staging: iio: adc: ad7192: Convert platform data to DT properties

This patch will remove platform data members and replace them with device
tree properties. These properties will be subject to further modifications
and probably replaced with other functionalities at some point in time.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>