msm: kgsl: Add gpu_frequency tracepoint to power trace system

Add a tracepoint to the power trace system to report GPU
frequency change. Format for this is identical to cpu_frequency
event and compatible with Android tools such as Perfetto.

Change-Id: I6207f45f506ab595bd261a297abdd8f40990c8de
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>