scmi: Introduce a new SCMI API and port CLK SCMI driver to it

Expose new scmi_buf_get/put API methods to build and send messages;
command request descriptors are now pre-allocated when the SCMI core is
initialized and kept in a free list, instead of being allocated on the
stack of the caller of the SCMI request.

Dynamically allocated descriptors enable the SCMI core to keep around
and track outstanding transactions for as long as needed, outliving the
lifetime of the caller stack: this allows tracking of late or missing
replies and it will be needed when adding support for SCMI transports
that allows for more messages to be inflight concurrently.

Move the existing CLK SCMI driver to the new API.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43046

scmi: Add SCMI message tracking and centralize tx/rx logic

In order to be able to support also new, more parallel, SCMI transports
that by nature can allow multiple concurrent commands to be in-flight,
pending a reply, we must be able to use the sequence number provided in
the SCMI messages to track the message status, matching commands and
replies while keeping track of timeouts and duplicates.

Add the needed message tracking machinery in the core SCMI stack and
move the residual common tx/rx logic from the specific transports to
the core SCMI stack, while adding one more interface to let the
transports customize ther behaviour.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43045

scmi: Protect SCMI/SMT channels from concurrent transmissions

The SCMI/SMT memory areas are used from the agent and the platform as
channels to exchage commands and replies.

Once the platform has completed its processing and a reply is ready to
be read from the agent, the platform will relinquish the channel to the
agent by setting the CHANNEL_FREE bits in the related SMT area.

When this happens, though, the agent has still to effectively read back
the reply message and any other concurrent request happened to have been
issued in the meantime will have been to be hold back until the reply
is processed or risk to be overwritten by the new request.

The base->mtx lock that currently guards the whole scmi_request()
operation is released when sleeping waiting for a reply, so the above
mentioned race can still happen or, in a slightly different scenario,
the concurrent transmission could just fail, finding the channel busy,
after having sneaked through the mutex.

Adding a new mechanism to let the agent explicitly acquire/release the
channel paves the way, in the future, to remove such central commmon
lock in favour of new dedicated per-transport locking mechanisms, since
not all transports will necessarily need the same level of protection.

Add a flag, controlled by the agent, to mark when the channel has an
inflight command transaction still pending to be completed and make the
agent spin on it when queueing multiple concurrent messages on the same
SMT channel.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43043

scmi: Fix SCMI mailbox polling mechanism

When the system is cold, the SCMI stack processes commands in polling
mode with the current polling mechanism being a check of the status
register in the mailbox controller to see if there is any pending
doorbell request.

Anyway, the completion interrupt is optional by the SCMI specification
and a system could have been simply designed without it: for this
reason polling on the mailbox controller status registers is not going
to work in all situations.

Moreover even alternative SCMI transports based on shared memory, like
SMC, will not have at all a mailbox controller to poll for.

On the other side, the associated SCMI Shared Memory Transport defines
dedicated channel flags and status bits that can be used by the agent to
explicitly request a polling-based transaction, even if the completion
interrupt was available, and to check afterwards when the platform has
completed its processing on the outstanding command.

Use SCMI/SMT specific mechanism to process transactions in polling mode.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43042

scmi: Extend and refactor SCMI shmem support

Add a few new common public scmi_shmem methods to be used to handle SCMI
shared memory areas from multiple transports; while doing that review
the shared memory accesses to read only the SMT header fields strictly
relevant to the SCMI message processing.

Move all the SCMI shmem related code to the existing scmi_shmem.c file
and add a new dedicated scmi_shmem.h header.

Introduce some commonly needed message header manipulation macros.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43041