mem: Split memory controller into base MemCtrl and HeteroMemCtrl

This change splits the default gem5 memory controller into two
memory controllers: MemCtrl (base memory controller which can be
used with only a single memory interface dram/nvm), and
HeteroMemCtrl (heterogeneous memory controller which inherits from
MemCtrl and requires a dram and an nvm memory interface).
New arguments are added to many of the base class (MemCtrl) functions
(for example memory inteface to use that function for) which helps
in easier use of these in the inherited class (HeteroMemCtrl).

Change-Id: Ifa4e9f9f1560c47063d1a8159a8c94add2e670bb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59731
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>

configs/common/MemConfig.py

@@ -235,7 +235,7 @@ def config_mem(options, system):
                 # Create a controller if not sharing a channel with DRAM
                 # in which case the controller has already been created
                 if not opt_hybrid_channel:
-                    mem_ctrl = m5.objects.MemCtrl()
+                    mem_ctrl = m5.objects.HeteroMemCtrl()
                     mem_ctrl.nvm = nvm_intf
 
                     mem_ctrls.append(mem_ctrl)