ARM: tegra: cpufreq thermal throttling cleanups

ARM: tegra: cpufreq thermal throttling cleanups

Various review feedback, including:

Keep a global throttling index that specifies a ceiling CPU speed, lowered by one
at each delay interval (while the temperature alarm continues to be signalled).
Avoid lowering the throttle too far based on a transitory lowering of speed
requested by the governor.

Restore governor-requested speed when throttling turned off.

Add cpufreq sysfs attribute for checking throttling state.

Make throttling workqueue high-priority.

Cosmetic changes.

Change-Id: I068bf32115927fa61282f17f4a8798f2aee0b530
Signed-off-by: Todd Poynor <toddpoynor@google.com>