Documentation: driver-api: PM: Add cpuidle document

Replace the remaining documents under Documentation/cpuidle/
with one more complete governor and driver API document for cpuidle
under Documentation/driver-api/pm/.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>