mem-ruby: Prevent ruby from crashing on CMOs

mem-ruby: Prevent ruby from crashing on CMOs

Ruby has no support for cache maintenace operations. As a workaround,
after printing a warning, we treat them as no-ops in the memory system
and respond immediately without handling them. There should be
workarounds in the memory system already that allow execution to
proceed without the requirement for cache maintenance operations.

Change-Id: I125ee4fa37b674c636d87f2d9205bbc1a74da101
Reviewed-on: https://gem5-review.googlesource.com/5057
Reviewed-by: Jieming Yin <bjm419@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>