PCI: iproc: Allow override of device tree IRQ mapping function

The iProc core PCIe driver defaults to using of_irq_parse_and_map_pci() for
IRQ mapping.  Add iproc_pcie.map_irq so bus interfaces that don't use
device tree can override this by supplying their own IRQ mapping function.

[bhelgaas: changelog]
Posting: http://lkml.kernel.org/r/1431465781-10753-1-git-send-email-hauke@hauke-m.de


Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ray Jui <rjui@broadcom.com.com>