dt-bindings: firmware: arm,scmi: Support mailboxes unidirectional channels

SCMI defines two kinds of communication channels between the agent and the
platform: one bidirectional 'a2p' channel used by the agent to send SCMI
commands and synchronously receive the related replies, and an optional
'p2a' unidirectional channel used to asynchronously receive delayed
responses and notifications emitted from the platform.

When configuring an SCMI transport based on mailboxes, the current binding
supports only mailboxes providing bidirectional channels: in such a case
one mailbox channel can be easily assigned to each SCMI channel as above
described.

In case, instead, to have to deal with mailboxes providing only distinct
unidirectional channels, it becomes necessary to extend the binding in
order to be able to bind 2 distinct unidirectional mailbox channels to the
same SCMI 'a2p' channel.

Bidirectional and unidirectional channels support for the SCMI mailbox
transport can coexist by carefully considering the effective combination
of defined 'mboxes' and 'shmem' descriptors.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230404115026.2828149-2-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

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