iio: dac: ad5770r: Add AD5770R support

The AD5770R is a 6-channel, 14-bit resolution, low noise, programmable
current output digital-to-analog converter (DAC) for photonics control
applications.

It contains five 14-bit resolution current sourcing DAC channels and one
14-bit resolution current sourcing/sinking DAC channel.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>