iio: dac: ad5686: Add support for AD5674R/AD5679R

The AD5674R/AD5679R are low power, 16-channel, 12-/16-bit buffered voltage
output digital-to-analog converters (DACs). They include a 2.5 V internal
reference (enabled by default).

These devices are very similar to AD5684R/AD5686R, except that they have 16
channels.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>