ARM: dts: msm: Add the parent-supply property to the GPU CX GDSC on SM8150

Cast an explicit vote on the CX rail on behalf of the GPU_CX GDSC
since not doing do might lead to the CX rail turning off whilst
the GDSC is still enabled. This in turn might lead to issues with
retention.

Change-Id: I5e165cb99220b391176c1599da0c09f5c2d50b4c
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>