mem-cache: Cleaned blocks should be marked as not writable

mem-cache: Cleaned blocks should be marked as not writable

A writeclean packet writes a dirty block to the memory below and
therefore sets the dirty flag for the block when the memory below is a
cache. If the block was also marked as writable it can satisfy future
write requests without further requests/snoops. This can lead to
multiple copies of the same block marked as dirty which is not
allowed. This changeset clears the writable flag from the cleaned
block to prevent the cache from satisfying future write requests
without sending a downstream request.

Change-Id: I14d3c62fd33f81b1a8ba62374c8565ccab00a6fe
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7821
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>