Input: ad7879 - convert SPI to dev_pm_ops

There is a move to deprecate bus-specific PM operations and move to
using dev_pm_ops instead in order to reduce the amount of boilerplate
code in buses and facilitate updates to the PM core. Do this move for
the ad7879 SPI driver.

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