dt-bindings: arm: Add virtio transport for SCMI

Document the properties for arm,scmi-virtio compatible nodes.
The backing virtio SCMI device is described in patch [1].

While doing that, make shmem property required only for pre-existing
mailbox and smc transports, since virtio-scmi does not need it.

[1] https://lists.oasis-open.org/archives/virtio-comment/202102/msg00018.html

Link: https://lore.kernel.org/r/[e-mail]
Co-developed-by: Peter Hilber <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Igor Skalkin <[e-mail]>
[ Peter: Adapted patch for submission to upstream. ]
Signed-off-by: Peter Hilber <[e-mail]>
[ Cristian: converted to yaml format, moved shmen required property. ]
Signed-off-by: Cristian Marussi <[e-mail]>
Signed-off-by: Sudeep Holla <[e-mail]>

Documentation/devicetree/bindings/firmware/arm,scmi.yaml

···
       - description: SCMI compliant firmware with ARM SMC/HVC transport
         items:
           - const: arm,scmi-smc
+      - description: SCMI compliant firmware with SCMI Virtio transport.
+                     The virtio transport only supports a single device.
+        items:
+          - const: arm,scmi-virtio
 
   interrupts:
     description:
···
       Each sub-node represents a protocol supported. If the platform
       supports a dedicated communication channel for a particular protocol,
       then the corresponding transport properties must be present.
+      The virtio transport does not support a dedicated communication channel.
 
     properties:
       reg:
···
 
 required:
   - compatible
-  - shmem
 
 if:
   properties:
···
 
   required:
     - mboxes
+    - shmem
 
 else:
   if:
···
   then:
     required:
       - arm,smc-id
+      - shmem
 
 examples:
   - |