drm/msm/dsi: Use generic PHY bindings

The DSI host links to the DSI PHY device using a custom binding. Switch to
the generic PHY bindings. The DSI PHY driver itself doesn't use the common
PHY framework for now.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>