ARM: dts: msm: Switch to use s6 corner regulator

Oxili GDSC was using non-corner s6 regulator as its parent
as the RPM was not ready for corner regulator. Since RPM
is supporting the corner regulator now, switch to use it.

Change-Id: Iec7d6e165368a1a05acaffe3d608acd0597ca07c
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>