ACPI: PCI: add a helper to convert _PRT INTx pin number to name

This adds a helper function to convert INTx pin numbers from the _PRT
(0, 1, 2, 3) to the pin name ('A', 'B', 'C', 'D').

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>