msm: kgsl: Add a GMU core function to read the always on counter

Add a GMU core function to read the always on counter from the generic
KGSL code.

Change-Id: Ic0dedbad1f81140ea9b891366d18b6f2d94bf149
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>