dt-bindings: arm: Add smc/hvc transport for SCMI

SCMI could use SMC/HVC as tranports. Since there is no standardized
SMC/HVC function identifier for the same, we need to use vendor
specific id. Add the same into the devicetree binding document.

Also add arm,scmi-smc compatible string for smc/hvc transport

Link: https://lore.kernel.org/r/1583673879-20714-2-git-send-email-peng.fan@nxp.comSigned-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
[sudeep.holla: reworded commit log and subject]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

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

@@ -14,7 +14,7 @@ Required properties:
 
 The scmi node with the following properties shall be under the /firmware/ node.
 
-- compatible : shall be "arm,scmi"
+- compatible : shall be "arm,scmi" or "arm,scmi-smc" for smc/hvc transports
 - mboxes: List of phandle and mailbox channel specifiers. It should contain
 	  exactly one or two mailboxes, one for transmitting messages("tx")
 	  and another optional for receiving the notifications("rx") if
@@ -25,6 +25,7 @@ The scmi node with the following properties shall be under the /firmware/ node.
 	  protocol identifier for a given sub-node.
 - #size-cells : should be '0' as 'reg' property doesn't have any size
 	  associated with it.
+- arm,smc-id : SMC id required when using smc or hvc transports
 
 Optional properties: