firmware: arm_scmi: Account for SHMEM memory overhead

Transports using shared memory have to consider the overhead due to the
layout area when determining the area effectively available for messages.

Till now, such definitions were ambiguos across the SCMI stack and the
overhead layout area was not considered at all.

Add proper checks in the shmem layer to validate the provided max_msg_size
against the effectively available memory area, less the layout.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20241028120151.1301177-2-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>