iio: adc: ad7923: Add of_device_id table

Accomplish device tree compatibility to driver AD7923
by adding of_device_id table and making a subsequent call to
MODULE_DEVICE_TABLE.

Signed-off-by: Daniel Junho <djunho@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>