[POWERPC] ibmebus: Prevent bus_id collisions

Previously, ibmebus derived a device's bus_id from its location code.
The location code is not guaranteed to be unique, so we might get bus_id
collisions if two devices share the same location code.  The OFDT
full_name, however, is unique, so we use that instead (truncating it
on the left if it is too long).

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