cpu: Add support for CMOs in the cpu models

cpu: Add support for CMOs in the cpu models

Cache maintenance operations go through the write channel of the
cpu. This changes makes sure that the cpu does not try to fill in the
packet with data.

Change-Id: Ic83205bb1cda7967636d88f15adcb475eb38d158
Reviewed-by: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5055
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>