arm64: dts: qcom: sm8450: add dp controller

Add the Display Port controller subnode to the MDSS node.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230206-topic-sm8450-upstream-dp-controller-v6-5-d78313cbc41d@linaro.org