Some nexus devices add wildcard children. Since fdtbus_probe returned
BUS_PROBE_DEFAULT, it would attach to them all, producing both many
fdtbus instances and preventing other devices from attaching. Instead
return BUS_PROBE_NOWILDCARD, which exists for exactly this purpose.
Since the PS3 port was committed, the AIM nexus device works perfectly fine
on all PowerPC platforms, whether or not they have Open Firmware. Remove
some more duplication and have there be only one nexus driver.