drm/msm/dsi: add support for DSI-PHY on SM8550

SM8550 use a 4nm DSI PHYs, which share register definitions
with 7nm DSI PHYs. Rather than duplicating the driver, handle
4nm variant inside the common 5+7nm driver.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/517515/
Link: https://lore.kernel.org/r/20230103-topic-sm8550-upstream-mdss-dsi-v3-6-660c3bcb127f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>