arm: DT autogeneration - autogenerate RealView Platform devices

Implements the Device Tree generating code for devices required by the
RealView VExpress_GEM5_V1 platform

Change-Id: I14244b2f3c028cbddba3c23ce7433fe3b301a0e8
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5965
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>