drm/msm/dsi: add support for 7nm DSI PHY/PLL

This adds support for the 7nm ("V4") DSI PHY/PLL for sm8150 and sm8250.

Implementation is based on 10nm driver, but updated based on the downstream
7nm driver.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> (SM8250)
Signed-off-by: Rob Clark <robdclark@chromium.org>