pwr: Adds logic to enter power gating for the cpu model

pwr: Adds logic to enter power gating for the cpu model

If the CPU has been clock gated for a sufficient amount of time
(configurable via pwrGatingLatency), the CPU will go into the OFF
power state.  This does not model hardware, just behaviour.

Change-Id: Ib3681d1ffa6ad25eba60f47b4020325f63472d43
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3969
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>