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: 6e986a8f ("dt-bindings: display: msm: Add binding for msm8998 dpu")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/497896/
Link: https://lore.kernel.org/r/20220817062059.18640-2-krzysztof.kozlowski@linaro.orgSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>