mfd: Add irq support in 88pm860x

88PM860x is a complex PMIC device. It contains touch, charger, sound, rtc,
backlight, led, and so on.

Host communicates to 88PM860x by I2C bus. Use thread irq to support this
usage case.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>