dev-arm: Carve out a portion of VExpress_GEM5 for the bootloader

dev-arm: Carve out a portion of VExpress_GEM5 for the bootloader

This patch is carving out a portion of VExpress_GEM5 memory for the
bootloader. Prior to this patch this was only happening
conditionally/dynamically via the setupBootLoader call.  With this patch
the region is always present and the setupBootLoader doesn't instantiate
memory, it is only setting up some bootloader parameters.

Change-Id: Iaa5cdf471b14e8faa37353a25631bf7c6fc64afc
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21604
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>