dev-arm: Automatically assign PCI device ids in attachPciDevice

dev-arm: Automatically assign PCI device ids in attachPciDevice

Simulation scripts currently need to assign PCI device addresses when
adding new devices. This change moves this responsibility to the
VExpress_GEM5_BASE::attachPciDevice method.

Change-Id: I6d62af8a8f9176d964cc011dd8fb9744154bbb87
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22830
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>