[ARM] mmp2: fix incorrect calling of chip->mask_ack() for 2nd level cascaded IRQs

The irq_chip is not yet registered, so no default irq_chip.mask_ack(),
which we have to handle it correctly manually here.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>