sim: Add a clear_parent function to NullSimObject.

Change-Id: I3842176f147997105fcc62aaf9cb93b9896708be
Reviewed-on: https://gem5-review.googlesource.com/4843
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/python/m5/params.py

@@ -1713,6 +1713,9 @@ class NullSimObject(object):
     def set_parent(self, parent, name):
         pass
 
+    def clear_parent(self, old_parent):
+        pass
+
     def __str__(self):
         return 'Null'