dev-arm: Fix pci_mem_base setting in VExpress_GEM5_Base

dev-arm: Fix pci_mem_base setting in VExpress_GEM5_Base

This was not actually used and DTB was generated using an hardcoded
value.

Change-Id: Ie8fd63495df5cb56418593cf0dd5432dc2992eac
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/+/27288
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>