dt-bindings: display/msm: document the SM8550 DSI PHY

Document the SM8550 DSI PHY which is very close from the 7nm
and 5nm DSI PHYs found in earlier platforms.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/517509/
Link: https://lore.kernel.org/r/20230103-topic-sm8550-upstream-mdss-dsi-v3-1-660c3bcb127f@linaro.org


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