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