thermal: cpuidle: Register cpuidle cooling device

The cpuidle driver can be used as a cooling device by injecting idle
cycles.

When the property is set, register the cpuidle driver with the idle
state node pointer as a cooling device. The thermal framework will do
the association automatically with the thermal zone via the
cooling-device defined in the device tree cooling-maps section.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20200429103644.5492-4-daniel.lezcano@linaro.org