MEM: Make the RubyPort physMemPort a PioPort instead of M5Port

MEM: Make the RubyPort physMemPort a PioPort instead of M5Port

This patch makes the physMemPort of the RubyPort a PioPort rather than
an M5Port. This reflects the fact that the M5Port and PioPort have
different roles. The M5Port is really a coherent slave that is
connected to the CPUs and other coherent masters of the system,
e.g. DMA ports. The PioPort, on the other hand, is a master port that
is connected to the memory and other slaves, for example the pio
devices.

This simplifies future changes into master/slave ports and is
consistent with the port roles throughout the system.