arm: DT autogeneration - Generate energy controller node
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 <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Curtis Dunham <14490907fdc9c150c92157b82121c81003bbdc71@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5966
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>

gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu