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.

Datasheets:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5672R_5676R.pdf
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5686R_5685R_5684R.pdf

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>