[POWERPC] pcibus_to_node fixes

of_node_to_nid returns -1 if the associativity cannot be found. This
means pcibus_to_cpumask has to be careful not to pass a negative index into
node_to_cpumask.

Since pcibus_to_node could be used a lot, and of_node_to_nid is slow (it
walks a list doing strcmps), lets also cache the node in the
pci_controller struct.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>