iio:dac:ad5686: Add AD5672R/76/76R/84R/86R support
The AD5684R/AD5686R are a family of 4 channel DACs with 12-bit, 14-bit and
16-bit precision respectively. The devices come either with a built-in
reference or no built-in reference.
The AD5672R/AD5676/AD5676R are similar, except that they have 8 channels
instead of 4.