iio: dac: ad5686: add of_match_table

Add of_match_table to this driver, so devices can be probed based on
device tree contents.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Link: https://lore.kernel.org/r/20200924195215.49443-2-michael.auchter@ni.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>