mem: Make MemCtrl a ClockedObject

mem: Make MemCtrl a ClockedObject

Made DRAMCtrl a ClockedObject, with DRAMInterface
defined as an AbstractMemory. The address
ranges are now defined per interface. Currently
the model only includes a DRAMInterface but this
can be expanded for other media types.

The controller object includes a parameter to the
interface, which is setup when gem5 is configured.

Change-Id: I6a368b845d574a713c7196c5671188ca8c1dc5e8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28968
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>