configs: arm realview(64) regressions using VExpress_GEM5_V1

configs: arm realview(64) regressions using VExpress_GEM5_V1

This patch is updating the arm regression configs so that the newer
VExpress_GEM_V1 platform is used instead of the older VExpress_EMM and
VExpress_EMM64.
A new optional kernel_mode argument has been added in order to
distinguish between realview and realview64 platforms. If not provided
the config will assume the machine is running a AArch64 kernel.

Other notable additions:
- DTB autogeneration in regressions
- Using minimal m5exit.squashfs disk image

Change-Id: Ia230565f072fe3eb7756c41876dba4657583f4df
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22687
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>