[ARM] mmp2: add handling on PMIC IRQ

Since PMIC INT pin is a special pin of CPU, the status of PMIC INT pin needs
to be cleared after PMIC IRQ occured. Now append the clear operation in
irq chip handler.

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