drm/msm/hdmi: Make HDMI core get its PHY

Make HDMI core get its PHY by parsing the "phys" phandle. The core will use
this PHY reference to enable/disable PHY. The driver defers probe until PHY
isn't available.

The DT bindings used here is the same as the one used for PHYs using the
common PHY framework bindings.

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