mem: Clean up Memory Controller

mem: Clean up Memory Controller

Make the actual controller more generic
    - Rename DRAMCtrl to MemCtrl
    - Rename DRAMacket to MemPacket
    - Rename dram_ctrl.cc to mem_ctrl.cc
    - Rename dram_ctrl.hh to mem_ctrl.hh
    - Create MemCtrl debug flag

Move the memory interface classes/functions to separate files
    - mem_interface.cc
    - mem_interface.hh

Change-Id: I1acba44c855776343e205e7733a7d8bbba92a82c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31654
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>