dt-bindings: display/msm: qcom, sa8775p-mdss: Add missing eDP phy

dt-bindings: display/msm: qcom, sa8775p-mdss: Add missing eDP phy

The Qualcomm SA8775p MDSS display block comes with eDP phy, already used
in DTS and already documented in phy/qcom,edp-phy.yaml binding.  Add the
missing device node in the binding and extend example to silence
dtbs_check warnings like:

  sa8775p-ride.dtb: display-subsystem@ae00000: Unevaluated properties are not allowed ('phy@aec2a00', 'phy@aec5a00' were unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Patchwork: https://patchwork.freedesktop.org/patch/638744/
Link: https://lore.kernel.org/r/20250221151311.138755-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>