MIPS: PCI: Update XLR/XLS PCI for the new PIC code

Use the nlm_set_pic_extra_ack() call to setup the extra interrupt
ACK needed by XLR PCI and XLS PCIe. Simplify the code by adding
nlm_pci_link_to_irq().

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4561
Signed-off-by: John Crispin <blogic@openwrt.org>