add support for enabling both the usb2 and usb3 phys.

add support for enabling both the usb2 and usb3 phys.

the code tried enabling the 0th phy in the usb-phy proplist, which
is the usb2 phy, and if that didn't exist it would try usb3-phy in
the standard phys/phy-names properties. it now tries to enable the
usb2 and usb3 phys independently.

further, support using standard phy drivers registered with the
ofw/fdt code, not just the ones handled inside the xhci driver.

ok kettenis@