ARM: dts: msm: Add the parent supply for CX GDSC for sdm670

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: I9fe10ab3e1198064e4d539e48d0be3be21c0ded6
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>