scmi: Add an SCMI VirtIO transport driver

Add an SCMI transport driver based on the virtio-scmi backend.

Reviewed by: andrew, bryanv
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43048

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: 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

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

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Add support for ARM System Control and Management Interface (SCMI) v3.1.

The SCMI specification describes a set of standard interfaces for power,
performance and system management.

SCMI is extensible and provides interfaces to access functions which are
often implemented in firmwares in the System Control Processor (SCP).

This implements Shared Memory-based transfer, which is one of the ways on
how messages are exchanged between agents and the platform.

This includes a driver for ARM Message Handling Unit (MHU) Doorbell, which
is a mechanism that the caller can use to alert the callee of the presence
of a message.

The support implements clock management interface. For instance this allows
us to control HDMI pixel clock on ARM Morello Board.

Tested on ARM Morello Board.

Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D37316
Reviewed by: manu
Sponsored by: UKRI