msm: mdss: add a separate register region for DSI PHY

The DSI PHY register offset from the controller base is
different for different targets. So, add a separate register
region for DSI PHY in the device tree. This register base and
offset will be used to access the DSI PHY registers. Also
change the register offsets which are dependent on the
controller base address.

Change-Id: Ie1d08950f3c8c8801908a1b3cf7db46a44b4e8c3
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>