iio: dac: ds4422/ds4424 dac driver

This patch provides an iio device driver for DS4422/DS4424 chips that support
two/four channel 7-bit Sink/Source Current DAC.

Signed-off-by: Ismail H. Kose <Ismail.Kose@maximintegrated.com>
Signed-off-by: Ismail H. Kose <ihkose@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>