ARM: dts: msm: Add OnSemi regulator node for msm8996

OnSemi buck regulator is used to provide the supply for
graphics rail. The programmable output voltage range is
from 0.6 V to 1.4 V in 6.25 mV steps.

Change-Id: I5f4ec11075b7f658ffa0af13dde5694b03c1495c
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>