dt-bindings: firmware: arm,scmi: Add support for powercap protocol

Add new SCMI v3.1 powercap protocol bindings definitions and example.

Link: https://lore.kernel.org/r/20220704102241.2988447-2-cristian.marussi@arm.com
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

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

@@ -183,6 +183,12 @@ properties:
             required:
               - reg
 
+  protocol@18:
+    type: object
+    properties:
+      reg:
+        const: 0x18
+
 additionalProperties: false
 
 patternProperties:
@@ -323,6 +329,10 @@ examples:
                     };
                 };
             };
+
+            scmi_powercap: protocol@18 {
+                reg = <0x18>;
+            };
         };
     };