iio: dac: Support ROHM BD79703 DAC

iio: dac: Support ROHM BD79703 DAC

The ROHM BD79703 is a 6 channel digital to analog converter.

Based on the data-sheet examples the hardware would support setting the
DAC word without changing the actual output. The data-sheet is not too
specific on how the enabling the output of new voltage set by DAC
should be done - hence this is not implemented by the driver.

The BD79703 would also support two specific "PULL_DOWN" modes. These
aren't currently supported by the driver either.

Add a very basic support for controlling the channel outputs one-by-one.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/bc77d7b979ca28408a216f597082fcd94ec63be7.1734608215.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>