mem: Add support for handling CMOs in the MSHRs

mem: Add support for handling CMOs in the MSHRs

To add support for cache maintenance operations (CMOs) in the MSHRs,
this change adds the following functionality:
- If a CMO request hits in the MSHRs, we deferred as we can't
  coalesce it with any other requests.
- When we promote any deferred targets, we promote them in order and
  stop if we encounter a CMO request. If the CMO request is at the
  beginning of the deferred targets list it will be the only promoted
  target.

Change-Id: I10d1f7e16bd6d522d917279c5d408a3f0cee4286
Reviewed-by: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5050
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>