arm64: introduce sys sd energy model infrastructure

Allow the energy model to contain a system level besides the already
existing core and cluster level.

This is necessary for platforms with frequency domains spanning all
cpus to let the EAS algorithm work properly.

The whole idea of this system level has to be rethought once
the idea of the 'struct sched_domain_shared' gets more momentum:

https://lkml.org/lkml/2016/6/16/209



Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>