msm: kgsl: Add GDSC support for dedicated power rail for GPU

There are two head switches for GPU, one is for the GPU core
itself, the other is for surrounding digital circuits. KGSL
needs to control them both.

The turn on sequence is:
1. Turn on GPU core
2. Turn on digital circuits

The turn off sequence is:
1. Turn off digital circuits
2. Turn off GPU core

Two regulator (actually head switch) names "vdd_dig" and "vdd"
are created in device tree file, representing digital
circuits and GPU core head switches respectively. In struct
kgsl_pwrctrl, adding a new regulator struct "gpu_dig"
which corresponds to vdd_dig. And the existing struct "gpu_reg"
corresponds to vdd.

Change-Id: Idb05caf547ad148885adb9cf498c47635067b70b
Signed-off-by: Pu Chen <puchen@codeaurora.org>