dt-bindings: iio: dac: Add docs for AD5770R DAC

Adding dt-bindings documentation for AD5770R DAC.

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>