mem: Ignore clean requests in the abstract memory

mem: Ignore clean requests in the abstract memory

Systems with atomic cores and the fastmem option enabled bypass the
whole memory system and access the abstract memory directly. Cache
maintenance operations which would be normally handled before the
point of unification/coherence should be ignored by the abstract
memory.

Change-Id: I696cdd158222e5fd67f670cddbcf2efbbfd5eca4
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5054
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>