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: Add new SCMI interfaces for init and message processing

Introduce a couple of new SCMI interface methods to allow centralized
initialization of transport-specific features and a couple of methods
to handle message reception from the SCMI core.

Move SCMI SMT related calls out of the core common SCMI code into the
transport specific layers Mailbox/SMC.

Make SCMI Mailbox/SMC transports use the new interface methods for
initialization and message reception.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43044

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: Split out the SCMI mailbox to a new file

Add a new SCMI interface file to allow for multiple kind of transports
and move the mailbox transport to its own file, using the new interface.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43039