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>

configs/example/arm/starter_fs.py

@@ -104,7 +104,8 @@ def create(args):
     # Only simulate caches when using a timing CPU (e.g., the HPI model)
     want_caches = True if mem_mode == "timing" else False
 
-    system = devices.SimpleSystem(want_caches,
+    system = devices.simpleSystem(LinuxArmSystem,
+                                  want_caches,
                                   args.mem_size,
                                   mem_mode=mem_mode,
                                   kernel=SysPaths.binary(args.kernel),