[POWERPC] Create of_buses for MV64x60 devices

For each mv64360 entry in the OpenFirmware database, add the
registration of an of_bus to take care of devices connected to
the MV64x60 asynchronous devices controller.
This change makes it possible for those devices to be detected by
drivers that support the of_platform without having a custom call
for each of them in the board file.

Signed-off-by: Remi Machet <rmachet@slac.stanford.edu>
Signed-off-by: Paul Mackerras <paulus@samba.org>