This patch adds pci_common_swizzle(), which swizzles INTx values all the
way up to a root bridge.

This common implementation can replace several architecture-specific
ones.  This should someday be combined with pci_get_interrupt_pin(),
but I left it separate for now to make reviewing easier.
Signed-off-by: NBjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>