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: 164f69d9d45a ("dt-bindings: msm: disp: add yaml schemas for QCM2290 DPU bindings")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>