MIPS: pci-ar724x: use per-controller IRQ base

Change to the code to use per-controller IRQ base.
This is needed for multiple PCI controller support.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4915/


Signed-off-by: John Crispin <blogic@openwrt.org>