dt-bindings: gpu: arm,mali-utgard: Add missing properties

Add common properties appearing in DTSes (opp-table) to fix dtbs_check
warnings like:

  arch/arm/boot/dts/exynos4210-i9100.dt.yaml: gpu@13000000:
    'opp-table' does not match any of the regexes: 'pinctrl-[0-9]+'
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200903191438.12781-2-krzk@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

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

@@ -100,6 +100,8 @@ properties:
 
   mali-supply: true
 
+  opp-table: true
+
   power-domains:
     maxItems: 1