um: virt-pci: implement pcibios_get_phb_of_node()

Implement pcibios_get_phb_of_node() as x86 does in order to allow PCI
busses to be associated with devicetree nodes.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>