arch-arm: Add support for automatic reset addr selection

arch-arm: Add support for automatic reset addr selection

Add an option to automatically set the aarch64 reset vector to the
entry point of the kernel. This is useful when running bare metal
workloads that don't use a normal boot loader.

Change-Id: Id472f865d461f0d8d8ea8efe5db582c170de0b90
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8143
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>