dt-bindings: display/msm: qcom, sm8250-mdss: add DisplayPort controller node

Document the DisplayPort controller node in MDSS binding, already used
in DTS:

  sm8250-xiaomi-elish-boe.dtb: display-subsystem@ae00000: Unevaluated properties are not allowed ('displayport-controller@ae90000' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Patchwork: https://patchwork.freedesktop.org/patch/566297/
Link: https://lore.kernel.org/r/20231107103600.27424-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>