msm: kgsl: track GPU active time per frequency

Tracks GPU active time per frequency for GPU workload
profiling. The data will be output in
/sys/class/kgsl/kgsl-3d0/gpu_clock_stats
with one u64 value in microseconds per clock level.
For example:
cat  /sys/class/kgsl/kgsl-3d0/gpu_clock_stats
39392 29292 929292 929292 4040404

CRs-Fixed: 1011462
Change-Id: I5f2caa8b38d99ffd23f03c1dfed1efda273fc2fb
Signed-off-by: George Shen <sqiao@codeaurora.org>