system-arm: Split the VExpress_GEM5_V1 base dts

With the introduction of the new DPU model, we need different
variations of the VExpress_GEM5_V1 platform. This splits the platform
dtsi file into a separate file for the base platform and the
HDLCD-based platform. This matches the hierarchy in RealView.py.

Change-Id: Id02380122655b5d3aa3548a703fdef178bba17d9
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11035
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

arm: Update dts to work with the new HDLCD driver

The dts files in system/arm/dt currently assume that an (unreleased)
gem5-specific virtual encoder is used as a remote endpoint for the
HDLCD. This driver won't be released as a more general virtual encoder
is about to be posted on the Linux DRI devel list and this encoder has
now been merged with gem5's kernel tree. This changeset updates gem5's
dts files to use that encoder.

Change-Id: Ic1a1be728efd31603752fdfba005b6dbdea42e7e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Rene De Jong <rene.dejong@arm.com>

arm: Ship Linux device trees with gem5

Ship aarch32 and aarch64 device trees with gem5. We currently ship
device trees as a part of the gem5 Linux kernel repository. This makes
tracking hard since device trees are supposed to be platform dependent
rather than kernel dependent (Linux considers device trees to be a
stable kernel ABI). It also makes code sharing between aarch32 and
aarch64 impossible.

This changeset implements a set of device trees for the new
VExpress_GEM5_V1 platform. The platform is described in a shared file
that is separate from the memory/CPU description. Due to differences
in how secondary CPUs are initialized, aarch32 and aarch64 use
different base files describing CPU nodes and the machine's
compatibility property.