ARM: dts: msm: Add the parent supply for CX GDSC for sdm845

For GDSCs that reside on domains that are collapsible and support
a minimum voltage of retention, explicitly vote on the minimal
operating voltage, LowSVS for the rail. Specifically this is
required for GPU_CX_GDSC for retaining the GPU resisters.

Change-Id: I346ebf92bf2a753f1c1ffa4cf38f08f5109c10b3
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>