dt-bindings: display: msm: sm8550-mdss: document displayport controller subnode

Document the optional displayport controller subnode of the SM8550 MDSS.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/544002/
Link: https://lore.kernel.org/r/20230621-topic-sm8x50-upstream-mdss-bindings-dp-subnode-v2-3-733ed383f6b6@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>