iio:dac:ad5686: Add AD5671R/75R/94/94R/95R/96/96R support

iio:dac:ad5686: Add AD5671R/75R/94/94R/95R/96/96R support

The AD5694/AD5694R/AD5695R/AD5696/AD5696R are a family of 4 channel DACs
with 12-bit, 14-bit and 16-bit precision respectively. The devices have
either no built-in reference, or built-in 2.5V reference.

The AD5671R/AD5675R are similar, except that they have 8 instead of 4
channels.

These devices are similar to AD5672R/AD5676/AD5676R and
AD5684/AD5684R/AD5684/AD5685R/AD5686/AD5686R, except that they use i2c
instead of spi.

Datasheets:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5671R_5675R.pdf
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5696R_5695R_5694R.pdf

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