thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver

thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver

The cpu idle cooling device offers a new method to cool down a CPU by
injecting idle cycles at runtime.

It has some similarities with the intel power clamp driver but it is
actually designed to be more generic and relying on the idle injection
powercap framework.

The idle injection duration is fixed while the running duration is
variable. That allows to have control on the device reactivity for the
user experience.

An idle state powering down the CPU or the cluster will allow to drop
the static leakage, thus restoring the heat capacity of the SoC. It
can be set with a trip point between the hot and the critical points,
giving the opportunity to prevent a hard reset of the system when the
cpufreq cooling fails to cool down the CPU.

With more sophisticated boards having a per core sensor, the idle
cooling device allows to cool down a single core without throttling
the compute capacity of several cpus belonging to the same clock line,
so it could be used in collaboration with the cpufreq cooling device.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20191219225317.17158-2-daniel.lezcano@linaro.org