PCI: designware: add ability for custom swizzle

Add the ability for a platform driver to provide a platform-specific
swizzle function.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
PCI: imx6: ventana: fixup for GW16082 IRQ mismapping

The TI XIO2001 PCIe-to-PCI bridge used on the Ventana GW16082 miniPCI
expansion mezzanine has its slot-to-bridge IRQ mapping reversed from the
PCI specification:

  INTA->INTD
  INTB->INTC
  INTC->INTB
  INTD->INTA

Implement a custom swizzle function that does a fixup on the interrupt for
devices behind a TI XIO2001 bridge.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>