ARM: dts: msm: Add support for controlling the GPU_GX GDSC on MSMSKUNK

Register the gpu_gx_gdsc with the regulator framework to allow
clients to toggle it. This support is temporary and is only
meant as a bridge until the firmware support is available.

CRs-Fixed: 1078731
Change-Id: I106e49705d2e35b3817a14bd609aa18bc83afdd5
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>