staging: iio: dac: ad5446: Enable driver support for AD5620/AD5640/AD5660 DA converters

Initial support for single channel, 12-/14-/16-Bit nanoDAC with On-Chip Reference

staging: iio: dac: ad5446: Fix according to review feedback

Review feedback by Jonathan Cameron:
	Use kernel doc style to document headers.
	Turn data into a union
	Add some comments for clarity

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>