[POWERPC] ibmebus: Add device creation and bus probing based on of_device

The devtree root is now searched for devices matching a built-in whitelist
during boot, so these devices appear on the bus from the beginning.  It is
still possible to manually add/remove devices to/from the bus by using the
probe/remove sysfs interface.  Also, when a device driver registers itself,
the devtree is matched against its matchlist.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>