MIPS: add irqdomain support for the CPU IRQ controller

Add code to load a irq_domain for the MIPS IRQ controller from a devicetree
file.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: David Daney <david.daney@cavium.com>
Patchwork: http://patchwork.linux-mips.org/patch/4902/