Input: ad714x-spi - convert to dev_pm_ops

There is a general move to convert drivers to use dev_pm_ops rather than
bus specific PM operations in order to facilitate core work. Do this
conversion for ad714x-spi.

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>