dt-bindings: arm: Add optional interrupt to smc/hvc SCMI transport

dt-bindings: arm: Add optional interrupt to smc/hvc SCMI transport

In the normal use of smc/hvc as SCMI transport, the message completion
is indicated by the return of the SMC call. This binding provides for
an optional interrupt named "a2p" which can be used instead to indicate
the completion of a message.

Link: https://lore.kernel.org/r/20201222145603.40192-2-jim2101024@gmail.com
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
[sudeep.holla: minor wording changes to the commit log]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>