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>

dev-arm: Add a VirtIO MMIO device to VExpress_GEM5_V1

Add an ARM-specific VirtIO MMIO device to the VExpress_GEM5_V1
platform.

Change-Id: Id1e75398e039aad9d637f46f653cda9084d3d2fe
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Sudhanshu Jha <sudhanshu.jha@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Rekai Gonzalez Alberquilla <rekai.gonzalezalberquilla@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2327

system-arm: Update gem5 timer interrupt specification

The DTB for the VExpress_GEM5_V1 was incorrectly flagging timer
interrupts as being edge triggered. Describe the interrupt as being
level triggered to match Juno and FVP.

Change-Id: I9ce4b8959e7cc28d8b208727119ff20e581311f8
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10024
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@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.