arm: DT autogeneration - Generate cpus node

arm: DT autogeneration - Generate cpus node

Equips cpu models with a method to generate the cpu node.

Note: even though official documentation requires that CPU ids start
counting from 0 in every cluster, GEM5 requires a globally unique cpu_id.

Change-Id: Ida3e17af3124a68ef7dbf2449cd034dfc3ec39df
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5963
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>