firmware: arm_scmi: Add support for unidirectional mailbox channels

Extend the SCMI transport layer to support mailbox controllers that expose
communication channels that are unidirectional by nature.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20230404115026.2828149-3-cristian.marussi@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>