mem: Split the various protocols out of the gem5 master/slave ports.

mem: Split the various protocols out of the gem5 master/slave ports.

This makes the protocols easier to see in their entirity, and makes it
easier to add a new type of port which only supports the functional
protocol.

Change-Id: If5d639bef45062f0a23af2ac46f50933e6a8f144
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20228
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>