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.comReviewed-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>

Documentation/devicetree/bindings/arm/arm,scmi.txt

@@ -31,6 +31,14 @@ Optional properties:
 
 - mbox-names: shall be "tx" or "rx" depending on mboxes entries.
 
+- interrupts : when using smc or hvc transports, this optional
+	 property indicates that msg completion by the platform is indicated
+	 by an interrupt rather than by the return of the smc call. This
+	 should not be used except when the platform requires such behavior.
+
+- interrupt-names : if "interrupts" is present, interrupt-names must also
+	 be present and have the value "a2p".
+
 See Documentation/devicetree/bindings/mailbox/mailbox.txt for more details
 about the generic mailbox controller and client driver bindings.