MIPS: Add common plat_irq_dispatch declaration

Add common declaration to get rid of following sparse warning: "symbol
'plat_irq_dispatch' was not declared. Should it be static?"

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Linux MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7539/


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