Some PPC SoCs have an EHCI with OHCI companion USB controller. To
emulate this allow the sysbus version of OHCI to be used as a companion.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>