msm: Add frequency backoff to thermal monitor

Make thermal monitor performance hit more gradual by stepping down
limit frequency instead of jumping directly to that frequency.
The monitor now steps down to the lowest available cpufreq
frequency, instead of fixing the limit frequency to 918MHz. Also
update the polling frequency to 250ms to improve responsiveness.

Change-Id: I6edb0cfc057284023978de04d7835e9783da5ebd
Signed-off-by: Eugene Seah <eseah@codeaurora.org>