mem: Add a WriteClean command to the packet class

mem: Add a WriteClean command to the packet class

A WriteClean packet allows a cache to write a block to a memory below
without evicting its copy. A typical usecase for a WriteClean packet
is a cache clean operation.

Change-Id: If356cb067da5ddf3210c135f41ef0891fb811568
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5044
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>