python: Don't assume SimObjects live in the global namespace

The importer in Python 3 doesn't like the way we import SimObjects
from the global namespace. Convert the existing SimObject declarations
to import from m5.objects. As a side-effect, this makes these files
consistent with configuration files.

Change-Id: I11153502b430822130722839e1fa767b82a027aa
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15981
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/arch/arm/ArmSystem.py

@@ -41,8 +41,8 @@ from m5.options import *
 from m5.SimObject import *
 from m5.util.fdthelper import *
 
-from System import System
-from ArmSemihosting import ArmSemihosting
+from m5.objects.System import System
+from m5.objects.ArmSemihosting import ArmSemihosting
 
 class ArmMachineType(Enum):
     map = {