mfd: MAX8998/LP3974 hibernation support

This patch makes the driver to save and restore register values
for hibernation.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>