MEM: Use base class Master/SlavePort pointers in the bus

MEM: Use base class Master/SlavePort pointers in the bus

This patch makes some rather trivial simplifications to the bus in
that it changes the use of BusMasterPort and BusSlavePort pointers to
simply use MasterPort and SlavePort (iterators are also updated
accordingly).

This change is a step towards a future patch that introduces a
separation of the interface and the structural port itself.