dt-bindings: display: msm: dp-controller: document SM8550 compatible

dt-bindings: display: msm: dp-controller: document SM8550 compatible

The SM8550 & SM8350 SoC shares the same DP TX IP version, use the
SM8350 compatible as fallback for SM8550.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/540552/
Link: https://lore.kernel.org/r/20230601-topic-sm8550-upstream-dp-v2-1-e8778109c757@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>