dt-bindings: display/msm: dpu-sc7180: add missing DPU opp-table

dt-bindings: display/msm: dpu-sc7180: 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: 62c0d21be463 ("dt-bindings: msm: disp: add yaml schemas for DPU bindings")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/497894/
Link: https://lore.kernel.org/r/20220817062059.18640-4-krzysztof.kozlowski@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>