MIPS: ralink: add PCI IRQ handling

The Ralink IRQ code was not handling the PCI IRQ yet. Add this functionaility
to make PCI work on rt3883.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5165/