iio:dac:ad5686: Add AD5311R support

AD5311R is a single channel DAC with 10-bit precision and i2c interface.
The device includes a 2.5V internal reference which is enabled by default.

This device is similar to AD5691R/AD5692R/AD5693/AD5693R, the only
difference is that it has 10-bit precision.

Datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5310R_5311R.pdf

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