arm: Ship Linux device trees with gem5

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.