drm/msm/hdmi: Create a separate HDMI PHY driver

Create a PHY device that represents the TX PHY and PLL parts of the HDMI
block.

This makes management of PHY specific resources (regulators and clocks)
much easier, and makes the PHY and PLL usable independently. It also
simplifies the core HDMI driver, which currently assigns phy ops among
many other things.

The PHY driver implementation done here is very similar to the PHY driver
we already have for DSI.

Keep the old hdmi_phy_funcs ops for now. The driver will use these until
the HDMI PHY/PLL register offsets aren't considered as separate
domains (i.e. their offsets start from 0).

The driver doesn't use the common PHY framework for now. This is because
it's hard to map our ops with the ops provided by the framework. The
bindings used for this is the generic phy bindings. So, this can be
adapted to the PHY framework in the future, if possible.

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