mem: Allow CMOs as snooping requests in the snoop filter

mem: Allow CMOs as snooping requests in the snoop filter

The snoop filter performs sanity checks of the type of packets that
are expected to snoop caches above. Cache maintenace operations are
expected to perform a clean and or invalidate on all caches down to
the specified point of reference and therefore could also generate
snoops.

Change-Id: I7f8fef246a85faa87ccd289c28b49686ed7caa08
Reviewed-by: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5052
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>