sim-power: Creation of PowerState class

sim-power: Creation of PowerState class

This commit does not make any functional changes but just rearranges
the existing code with regard to the power states. Previously, all
code regarding power states was in the ClockedObjects. However, it
seems more logical and cleaner to move this code into a separate
class, called PowerState. The PowerState is a now SimObject. Every
ClockedObject has a PowerState but this patch also allows for objects
with PowerState which are not ClockedObjects.

Change-Id: Id2db86dc14f140dc9d0912a8a7de237b9df9120d
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28049
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>