PCI: Add IRQ mapping function pointers to pci_host_bridge struct

In order to defer IRQ assignment arches must be able to register functions
to map and swizzle interrupts.  These registered functions are stored in
the pci_host_bridge struct.

Signed-off-by: Matthew Minter <matt@masarand.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>