drm/msm/a6xx: Add devfreq support for a6xx

Implement routines to estimate GPU busy time and fetching the
current frequency for the polling interval. This is required by
the devfreq framework which recommends a frequency change if needed.
The driver code then tries to set this new frequency on the GPU by
sending an Out Of Band(OOB) request to the GMU.

Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>