MIPS/PCI: Implement pcibios_get_phb_of_node

The of_node field of the device assigned to a
PCI bus is used during scanning of the PCI bus.
However on MIPS, the of_node field is assigned
only after the bus has been scanned.

Implement the architecture specific version of
'pcibios_get_phb_of_node'. Which ensures that the
PCI driver core will initialize the of_node field
before starting the scan.

Also remove the local assignment of bus->dev.of_node,
it is not needed after the patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>