mfd: Add ability to wake the system for 88pm860x

For 88pm860x pmic, it can wake the system from low power mode by irq,
its sub-devs like RTC and onkey can be enabled for this usage.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>