mem: Put gem5 protocols in their own directory.

This reduces clutter in the src/mem directory, and makes it clear that
those protocols are for the classic gem5 memory system, not ruby, TLM,
etc.

Change-Id: I6cf6b21134d82f4f01991e4fe92dbea8c7e82081
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20231
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>