iio: dac: ad5446: Add support for new AD5600 DAC

iio: dac: ad5446: Add support for new AD5600 DAC

The AD5600 is a single channel, 16-bit resolution, voltage output digital
to analog converter (DAC). The AD5600 uses a 3-wire SPI interface. It is
part of the AD5541 family of DACs.

The ad5446 IIO driver implements support for some of these DACs (in the
AD5441 family), so the change is a simple entry in this driver.

Link: https://www.analog.com/media/en/technical-documentation/data-sheets/AD5600.pdf
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>