arch-arm: Switch to DTOnly as the default machine type

arch-arm: Switch to DTOnly as the default machine type

Old ARM systems used to pass the machine type in the ATAGS list passed
to the kernel. This has been largely deprecated by the introduction of
device trees. Switch to the DTOnly machine type by default in gem5
since all new platforms and kernel will require this behavior.

Change-Id: Icfd085e4862863b4ef495566bfddbd11591866c3
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4260
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>