PM / devfreq: tegra30: Support thermal cooling

Expose ACTMON devfreq device as a cooling device in order to throttle
memory freq on overheat. Throttling of memory freq has a significant
cooling effect on NVIDIA Tegra SoCs since higher memory freqs require
higher SoC core voltage which is one of the main causes of the heating.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>