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