mem-ruby: sequencer callback for unique writes

mem-ruby: sequencer callback for unique writes

A controller may complete a write without obtaining a full copy of
the line. This patch adds a specific callback for this purpose that
prevents reads to be coalesced with a write on a potentially incomplete
line.

Change-Id: I3775f81699f38e406fee28f92c9c8e06deb3d528
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31269
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bradford Beckmann <bradford.beckmann@gmail.com>