configs: Add simpleSystem helper to generate devices.SimpleSystem

configs: Add simpleSystem helper to generate devices.SimpleSystem

This patch will make it possible to generate a SimpleSystem inheriting
from a configurable base class. More practically it will be possible to
inherit from a baremetal System (ArmSystem) rather than from a
LinuxArmSystem

Change-Id: I11553ae8045519059e159c555c6c9141bb4519b7
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/+/21603
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>