MIPS: lantiq: rework external irq code

This code makes the irqs used by the EIU loadable from the DT. Additionally we
add a helper that allows the pinctrl layer to map external irqs to real irq
numbers.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4818/