dt-bindings: iio: dac: Document AD5446 and similar devices

dt-bindings: iio: dac: Document AD5446 and similar devices

Add device tree binding documentation for the Analog Devices AD5446
family of Digital-to-Analog Converters and derivative devices from
Texas Instruments. There's both SPI and I2C interfaces and feature
resolutions ranging from 8-bit to 16-bit.

The binding covers 29 derivatives devices including the AD5446 series,
AD5600 series, AD5620/5640/5660 variants with different voltage ranges,
and TI DAC081s101/DAC101s101/DAC121s101 devices.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>