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