Added PCI_BUS_NUM_RESOURCES as Ben suggested. Default value is 4
and can be overridden by arch (probably in asm/system.h).
pci_read_bridge_bases() and pci_assign_bus_resource() changed
accordingly. "for (i = 0 ; i < 4; i++)" in pci_add_new_bus() not
changed, as it's used _only_ for pci-pci and cardbus bridges.