mem-cache: Move tagsInUse to children

mem-cache: Move tagsInUse to children

Move tagsInUse to children, as sector caches have different
tag invalidation and insertion, and thus they must handle
updating this variable.

Change-Id: I875c9b7364a909c76daf610d1e226c4e82063870
Reviewed-on: https://gem5-review.googlesource.com/10721
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>