arch-arm: generic method for getting an ArmSystem

arch-arm: generic method for getting an ArmSystem

This patch provides a generic method for casting a System object
into an ArmSystem object. This is specially useful in dev-arm,
since devices by default obtain a generic System reference which
needs to be casted to use ArmSystem-specific functionality.

Change-Id: Ib100002413cb48cd93772dcf38f13be65badd1d3
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22426
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>