dt-bindings: display/msm: qcom, sdm845-mdss: document the DP device

Document the DP controller added to the sdm845 display subsystem.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/517447/
Link: https://lore.kernel.org/r/20230109045458.316114-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>