msm: kgsl: Create a6xx rgmu power ops

This completely dissociates the power sequences from
the kgsl power controller for rgmu based targets and
makes them target specific.

Change-Id: I7f848ab507762ee6cdcfa6f64d07c84967a35bfa
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>