mem: Factor out DRAM interface

mem: Factor out DRAM interface

Separate out the DRAM specific functions into a new class:
- DRAMInterface

Move the Rank class to DRAMInterface since rank functions are
specific to the media.

Move DRAMPacket and Bank class definitions outside of DRAMCtrl.

This is patch 1 within a set of patches that separate the
memory controller and media interface. These changes enable
future incorporation of new media interfaces into the model.

Change-Id: Ief632d73ec27176a61f16b4f642c10d56630f376
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28967
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>