msm: kgsl: Initial implementation of A6XX RGMU driver

Add support for initial implementation of A6xx RGMU
driver. Change adds new device tree node and other
functionalities such as probe, start, stop, etc.
RGMU is a reduced Graphics Management Unit which
performs GPU power collapse.

Change-Id: I72186fa4cbf10cf6d612940af29d93872ee9649c
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>