drivers:iio:dac: Add AD3552R driver support

The AD3552R-16 is a low drift ultrafast, 16-bit accuracy,
current output digital-to-analog converter (DAC) designed
to generate multiple output voltage span ranges.
The AD3552R-16 operates with a fixed 2.5V reference.

Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/ad3552r.pdf

Signed-off-by: Mihail Chindris <mihail.chindris@analog.com>
Link: https://lore.kernel.org/r/20211213110825.244347-3-mihail.chindris@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>