Input: ad7879 - convert I2C to dev_pm_ops

There is a general move towards the use of dev_pm_ops rather than
bus specific suspend APIs as this simplifies both the bus and PM core
implementations. Convert the ad7879-ts I2C support over.

Compile tested only.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>