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.

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>