ARM: dts: msm: keep refgen regulator always-on for SDM845 v2

Ensure that the South refgen regulator is always enabled.  This
is needed so that DSI (display) and CSI (camera) PHYs continue to
operate correctly before they add explicit requests for the
refgen regulator.

Change-Id: I97f026ac6ba30bca6f0fbba5820c4471360f9f98
Signed-off-by: David Collins <collinsd@codeaurora.org>