iio:dac:ad5686: Add AD5691R/AD5692R/AD5693/AD5693R support

The AD5691R/AD5692R/AD5693/AD5693R are a family of one 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.

These devices are pretty similar to AD5671R/AD5675R and
AD5694/AD5694R/AD5695R/AD5696/AD5696R, except that they have one channel.
Another difference is that they use a write control register(addr 0x04) for
setting the power down modes and the internal reference instead of separate
registers for each function.

Datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5693R_5692R_5691R_5693.pdf



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