iio: adc: ad799x: add pm_ops to disable the device completely

iio: adc: ad799x: add pm_ops to disable the device completely

The device is always in a low-power state due to the hardware design. It
wakes up upon a conversion request and goes back into the low-power
state. The pm ops are added to disable the device completely and to free
the regulator. Disbaling the device completely should be not that
notable but freeing the regulator is important. Because if it is a shared
power-rail the regulator won't be disabled during suspend-to-ram/disk
and so all devices connected to that rail keeps on.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>