ARM: dts: msm: add refgen regulator device for sdm670

Add a refgen-regulator device for the South refgen used by
the CSI and DSI PHYs.

Also keep the South refgen regulator always on until the
DSI (display) and CSI (camera) clients add their requests
for the refgen regulator.

CRs-Fixed: 2113021
Change-Id: I31a5fcc62263607616589c9bd6ed79dc6b863f22
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>