arm: DT autogeneration - Generate energy controller node

Adds Device Tree methods for the energy controller to allow for
DVFS simulations with automatically generated DTB files

Change-Id: Id8682f07dff1bbe63987e757faa0694e03ee86ab
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5966
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>