The NPCM730 and NPCM750 chips have a single USB host port shared between
a USB 2.0 EHCI host controller and a USB 1.1 OHCI host controller. This
adds support for both of them.
Testing notes:
* With -device usb-kbd, qemu will automatically insert a full-speed
hub, and the keyboard becomes controlled by the OHCI controller.
* With -device usb-kbd,bus=usb-bus.0,port=1, the keyboard is directly
attached to the port without any hubs, and the device becomes
controlled by the EHCI controller since it's high speed capable.
* With -device usb-kbd,bus=usb-bus.0,port=1,usb_version=1, the
keyboard is directly attached to the port, but it only advertises
itself as full-speed capable, so it becomes controlled by the OHCI
controller.
In all cases, the keyboard device enumerates correctly.