mfd: Convert aat2870 to dev_pm_ops

The I2C suspend and resume functions have been deprecated since the driver
was introduced.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jin Park <jinyoungp@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>