MIPS: lantiq: split up IRQ IM ranges

Up to now all our SoCs had the 5 IM ranges in a consecutive order. To accomodate
the SVIP we need to support IM ranges that are scattered inside the register range.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4237/