mem: Minimize the use of MemObject.

MemObject doesn't provide anything beyond its base ClockedObject any
more, so this change removes it from most inheritance hierarchies.
Occasionally MemObject is replaced with SimObject when I was fairly
confident that the extra functionality of ClockedObject wasn't needed.

Change-Id: Ic014ab61e56402e62548e8c831eb16e26523fdce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18289
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/dev/Device.py

@@ -43,9 +43,9 @@ from m5.params import *
 from m5.proxy import *
 from m5.util.fdthelper import *
 
-from m5.objects.MemObject import MemObject
+from m5.objects.ClockedObject import ClockedObject
 
-class PioDevice(MemObject):
+class PioDevice(ClockedObject):
     type = 'PioDevice'
     cxx_header = "dev/io_device.hh"
     abstract = True