MIPS: BCM47XX: add asmlinkage to plat_irq_dispatch()

plat_irq_dispatch() is called from asm code, add asmlinkage.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6043/