staging: iio: ad9834: add of_device_id table

Add a of_device_id struct array of_match_table variable and subsequent
call to MODULE_DEVICE_TABLE macro to enable explicit device tree support.

Co-developed-by: Thiago L. A. Miller <tmiller@mochsl.org.br>
Signed-off-by: Thiago L. A. Miller <tmiller@mochsl.org.br>
Co-developed-by: Osvaldo M. Yasuda <omyasuda@yahoo.com.br>
Signed-off-by: Osvaldo M. Yasuda <omyasuda@yahoo.com.br>
Signed-off-by: João Victor Marques de Oliveira <joao.marques.oliveira@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>