ARM: dts: msm: add 20nm DSI PHY specific settings for 8994

Update 20nm PHY regulator and strength control settings.
Add the needed regulator for 20nm DSI Physical layer.
Fix the dsi cell index to support dual DSI on 8994. Add
the default resolution for writeback panel.

Change-Id: I247027ca35ac05737b599b85eed5cbdeeb51d353
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>