ARM: dts: msm: Add parent supply for oxili gx gdsc device

S6 regulator is the parent supply of the oxili gx gdsc
device. It needs to be turned on when the oxili gx gdsc
is turned on. Therefore, add the s6 regualtor as the
parent supply.

Note that after the s6 control switch to rpm, the s6
corner regulator will be used as the parent supply.

Change-Id: Ia4a7507d47ce08afbbc492c88f394d5878f3302d
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>