dt-bindings: gpu: mali: Add optional OPPs

The operating-points-v2 binding gives a way to provide the OPP of the GPU.
Let's use it.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>

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

@@ -39,6 +39,10 @@ Optional properties:
     Memory region to allocate from, as defined in
     Documentation/devicetree/bindi/reserved-memory/reserved-memory.txt
 
+  - operating-points-v2:
+    Operating Points for the GPU, as defined in
+    Documentation/devicetree/bindings/opp/opp.txt
+
 Vendor-specific bindings
 ------------------------