[POWERPC] Fix bus probe on Bamboo board

Commit 804ace88 changed the behavior of how compatible nodes are found.
This highlighted a bug on the Bamboo board where it wasn't probing the bus
specified in the DTS file.  We fix it by being explicit about which bus to
probe.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: David Gibson <david@gibson.dropbvear.id.au>