mem: Add a MemBackdoor type to track memory backdoors.

These are similar to the structures TLM's DMI mechanism uses. Instead
of having an invalidation broadcast which propogates backwards up the
port hierarchy, this mechanism tracks a set of callbacks which are
triggered when a back door is invalidated to let other holders clean
up their bookkeeping.

Change-Id: If24489258dcaee14d7b6e5b996dfb1c2636f26ab
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17589
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>