mem: Move ruby protocols into a directory called ruby_protocol.

Now that the gem5 protocols are split out, it would be nice to put them
in their own protocol directory. It's also confusing to have files
called *_protocol which are not in the protocol directory.

Change-Id: I7475ee111630050a2421816dfd290921baab9f71
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20230
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

learning_gem5: Add a simple Ruby protocol

Adds the MSI protocol from "A Primer on Memory Consistency and Cache
Coherence" by Daniel J. Sorin, Mark D. Hill, and David A. Wood.

This code follows Learning gem5 Part 3.
http://learning.gem5.org/book/part3/index.html

This is meant to be a simple, clean, example of how to make a Ruby
protocol.
Currently, it only works in SE mode.

The next changeset will contain the required configuration files.

Change-Id: If2cc53f5e6b9c6891749f929d872671615a2b4ab
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/8942