MIPS: SB1250: Restore dropped irq_mask function

Commit d6d5d5c4 (MIPS: Sibyte: Convert to new irq_chip functions)
removed the mask function which breaks irq_shutdown(). Restore it.

Reported-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2460/


Tested-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>