iio: dac: ad5686: add support for AD5338R

iio: dac: ad5686: add support for AD5338R

The AD5338R is a 10-bit DAC with 2 outputs and an internal 2.5V
reference (enabled by default). The register configuration is nearly
identical to the AD5696R DAC that's already supported by this driver,
with the channel selection bits being the only thing different.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Link: https://lore.kernel.org/r/20200924195215.49443-1-michael.auchter@ni.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>