dt-bindings: gpu: mali-utgard: add optional power-domain reference

On some socs Mali Utgard gpus have both soc power-domains and external
supplying regulators, so add an optional power-domains property for
reference.
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>
Acked-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/gpu/arm,mali-utgard.txt

@@ -52,6 +52,10 @@ Optional properties:
     Operating Points for the GPU, as defined in
     Documentation/devicetree/bindings/opp/opp.txt
 
+  - power-domains:
+    A power domain consumer specifier as defined in
+    Documentation/devicetree/bindings/power/power_domain.txt
+
 Vendor-specific bindings
 ------------------------