staging: IIO: DAC: AD5446: Add support for AD5601/AD5611/AD5621

This patch adds support for the AD5601/AD5611/AD5621 single channel,
8-/10-/12-bit, buffered voltage output DACs.

Changes since v1:
Sort Kconfig description my number

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>