spapr_pci: Add numa node id

This adds a numa id property to a PHB to allow linking passed PCI device
to CPU/memory. It is up to the management stack to do CPU/memory pinning
to the node with the actual PCI device.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[dwg: Renamed property from "node" to "numa_node" to match the similar
 one in the pxb device]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>