MIPS: ralink: adds irq code

All of the Ralink Wifi SoC currently supported by this series share the same
interrupt controller (INTC).

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4890/