MIPS: BCM47XX: add vectored interrupt support

This adds support for vectored interrupt which is supported by the SoC
using a MIPS 74K CPU like the BCM4716 and BCM4706.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6290/