ACPI pci-to-pci bridges

ACPI pci-to-pci bridges

- every pci device which implements _PRT acpi method is considered to
  be a pci-to-pci bridge

- acpi driver constructs a hierarchy of pci-to-pci bridges

- when pci driver identifies a pci-to-pci bridge it tells acpi driver
  what is the primary and the secondary bus for this device

- when pci requests IRQ routing information from acpi, it passes the
  bus number too to be able to identify the device accurately