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/dev/Device.py

@@ -42,7 +42,8 @@
 from m5.params import *
 from m5.proxy import *
 from m5.util.fdthelper import *
-from MemObject import MemObject
+
+from m5.objects.MemObject import MemObject
 
 class PioDevice(MemObject):
     type = 'PioDevice'