ARM: dts: msm: Adding Refgen vote for DSI Ctrl

This change adds Refgen supply for DSI Controller. This 
enables us to vote for Refgen supply in the software.

Change-Id: I06d1a87be2536dc985cbbb0386a2844626a5975a
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>