ARM: dts: msm: Add minimum CX vote for GPU CX GDSC on Kona

Add the vdd_parent supply for GPU CX GDSC to ensure minimal CX vote when
the GDSCR is enabled, thus this would ensure the CX cannot go
lower/collapse when GPU CX GDSCR is enabled.

Change-Id: I40f20b9e22d22e40a35049ebd19e57bc319d1334