learning_gem5: Add a simple config for MI_example

learning_gem5: Add a simple config for MI_example

Adds a new config script to configure the MI_example protocol. This script
closely follows the script used for MSI, but instead supports the
MI_example protocol. This script works with the simple_ruby runscript and
can be included instead of msi_caches.

Change-Id: I8be0be67bf51369763ba103a5f101cfc01ad8859
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/8945
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>