mem: Move the Port base class into sim.

mem: Move the Port base class into sim.

The Port class is going to be officially used for more than just memory
system connections.

Change-Id: I493e721f99051865c5f0c06946a2303ff723c2af
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17036
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>