iio: dac: Add AD5758 support

The AD5758 is a single channel DAC with 16-bit precision which uses the
SPI interface that operates at clock rates up to 50MHz.

The output can be configured as voltage or current and is available on a
single terminal.

Datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/ad5758.pdf

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