arm64: dts: qcom: sm8550: add display port nodes

Add the Display Port controller subnode to the MDSS node.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230601-topic-sm8550-upstream-dp-v4-2-ac2c6899d22c@linaro.org