msm thermal: enhancements

1) Smart polling based on temp (poll slower when cooler, faster when
warmer).

2) Added low frequency index so we don't throttle too low and kill
performance. When warming normally (throttle temp is reached) it will
step down by the freq_step until back under control. The lowest
frequency step possible is the specified by the low freq index.

3) Added throttle max (hardcoded at 80C). If it reaches the
throttle max, it will immediately throttle down to the min_freq_index
instead of stepping down 2 frequency steps at a time like when it hits
the throttle temp.

4) Added tuneables

/sys/module/msm_thermal/parameters/throttle_temp
/sys/module/msm_thermal/parameters/min_freq_index
/sys/module/msm_thermal/parameters/thermal_debug