[POWERPC] Check for offline nodes in pci NUMA code

During boot we bring up all memory and cpu nodes. Normally a PCI device
will be in one of these online nodes, however in some weird setups it
may not.

We have only seen this in the lab but we may as well check for the case
and fallback to -1 (all nodes).

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