This patch changes the way xhci ports are linked to USBPorts.  The fixed
1:1 relationship between xhci ports and USBPorts is gone.  Now each
USBPort represents a physical plug which has usually two xhci ports
assigned: one usb2 and ond usb3 port.  usb devices show up at one or the
other, depending on whenever they support superspeed or not.

This patch also makes the number of usb2 and usb3 ports runtime
configurable by adding 'p2' and 'p3' properties.  It is allowed to
have different numbers of usb2 and usb3 ports.  Specifying p2=4,p3=2
will give you an xhci adapter which supports all speeds on physical
ports 1+2 and usb2 only on ports 3+4.