dt-bindings: display: msm: sm6350-mdss: document DP controller subnode

Document the displayport controller subnode of the SM6350 MDSS.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/585468/
Link: https://lore.kernel.org/r/20240329-sm6350-dp-v2-2-e46dceb32ef5@fairphone.com


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