MEM: Add the system port as a central access point

MEM: Add the system port as a central access point

The system port is used as a globally reachable access point to the
memory subsystem. The benefit of using an actual port is that the
usual infrastructure is used to resolve any access and thus makes the
overall system able to handle distributed memories in any
configuration, and also makes the accesses agnostic to the address
map. This patch only introduces the port and does not actually use it
for anything.