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>

src/sim/SConscript

@@ -54,6 +54,7 @@ Source('global_event.cc')
 Source('init.cc', add_tags='python')
 Source('init_signals.cc')
 Source('main.cc', tags='main')
+Source('port.cc')
 Source('root.cc')
 Source('serialize.cc')
 Source('drain.cc')