staging: iio: ad7746: add device tree support

Add a of_device_id struct variable and subsequent call to
MODULE_DEVICE_TABLE macro to support device tree.

Signed-off-by: João Seckler <joaoseckler@gmail.com>
Signed-off-by: Lucas Oshiro <lucasseikioshiro@gmail.com>
Co-developed-by: Lucas Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>