MIPS: BCM63XX: add external irq support for BCM6345

Add the missing definitions for BCM6345.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4091/


Signed-off-by: John Crispin <blogic@openwrt.org>