dt-bindings: display/msm: dpu-sc7280: add missing DPU opp-table

The 'display-controller' child (DPU) of Display SubSystem (MDSS) uses
opp-table, so reference it which allows restricting DPU schema to fixed
list of properties.

Fixes: 57fd4f34ddac ("dt-bindings: msm: add DT bindings for sc7280")
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/497898/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Baryshkov <[e-mail]>
Signed-off-by: Rob Clark <[e-mail]>

Documentation/devicetree/bindings/display/msm/dpu-sc7280.yaml

···
   "^display-controller@[0-9a-f]+$":
     type: object
     description: Node containing the properties of DPU.
+    additionalProperties: false
 
     properties:
       compatible:
···
         maxItems: 1
 
       operating-points-v2: true
+      opp-table:
+        type: object
 
       ports:
         $ref: /schemas/graph.yaml#/properties/ports