PCI: Provide a default pcibios_update_irq()

Most architectures implement this in exactly the same way. Instead of
having each architecture duplicate this function, provide a single
implementation in the core and make it a weak symbol so that it can be
overridden on architectures where it is required.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>