When using more than one PCI bus, we have to assign unique numbers
to each bus. Use an auto-incremented bus index and assign it to
each registered bus. Also, allow the PCI host controller to update
internal registers by calling set_busno with assigned bus number.

While at it, add pci_controller struct to set_busno callback,
add a back reference to pci_controller to pci_bus, and clean up
unused left-overs from Linux import.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>