dt-bindings: display: msm: dp-controller: document SM8250 compatible

It looks like DP controlled on SM8250 is the same as DP controller on
SM8350. Use the SM8350 compatible as fallback for SM8250.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/546242/
Link: https://lore.kernel.org/r/20230709041926.4052245-2-dmitry.baryshkov@linaro.org