ARM: dts: msm: Add minimum CX vote for GPU CX GDSC on SM8150

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: Ic75c1380097a5e25feadbd4cb52d289e32e332b2
Signed-off-by: Jagadeesh Kona <jkona@codeaurora.org>