MIPS: BCM63XX: use a switch for external irq config

Makes the code a bit more readable and easier to add support for
new chips.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4093/
Signed-off-by: John Crispin <blogic@openwrt.org>