mem: Add NVM interface

mem: Add NVM interface

Add NVM interface to memory controller.

This can be used with or instead of the existing
DRAM interface. Therefore, a single controller can interface
to either DRAM or NVM, or both.

Specifically, a memory channel can be configured as:
- Memory controller interfacing to DRAM only
- Memory controller interfacing to NVM only
- Memory controller interfacing to both DRAM and NVM

How data is placed or migrated between media types is outside
of the scope of this change.

The NVM interface incorporates new static delay parameters
for read and write completion. The interface defines a 2
stage read to manage non-deterministic read delays while
enabling deterministic data transfer, similar to NVDIMM-P.
The NVM interface also includes parameters to define
read and write buffers on the media side (on-DIMM). These are
utilized to quickly offload commands and write data, mitigating
the effects of lower latency and bandwidth media characteristics.

Change-Id: I6b22ddb495877f88d161f0bd74ade32cc8fdcbcc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29027
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>