MIPS: BCM63XX: Handle SW IRQs 0-1

MIPS software IRQs 0 and 1 are used for interprocessor signaling (IPI)
on BMIPS SMP.  Make the board support code aware of them.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
[jogo@openwrt.org: move sw irqs behind timer irq]
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Patchwork: https://patchwork.linux-mips.org/patch/5354/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>