firmware: scmi: mailbox/smt agent device

firmware: scmi: mailbox/smt agent device

This change implements a mailbox transport using SMT format for SCMI
exchanges. This implementation follows the Linux kernel and
SCP-firmware [1] as references implementation for SCMI message
processing using SMT format for communication channel meta-data.

Use of mailboxes in SCMI FDT bindings are defined in the Linux kernel
DT bindings since v4.17.

Links: [1] https://github.com/ARM-software/SCP-firmware
Signed-off-by: Etienne Carriere <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Peng Fan <[email protected]>
Cc: Sudeep Holla <[email protected]>
Reviewed-by: Simon Glass <[email protected]>