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