mem: Add the notion of point of unification in the coherent xbar

mem: Add the notion of point of unification in the coherent xbar

The point of unification is the first crossbar at which the
instruction cache, the data cache and the translation table walks of
the core are guaranteed to see the same copy of a memory location.

Change-Id: Ica79b34c8ed4f1a8f2379748e8520a8f8afffa90
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5040
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>