staging: iio: adis16240: add of_match_table entry

This patch adds of_match_table entry in device driver in order to
enable spi fallback probing.

Signed-off-by: Rodrigo Ribeiro <rodrigorsdc@gmail.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>