iio: dac: ad5446: Add support for AD5641

This patch adds support for the AD5641 single channel,
14-bit, buffered voltage output DAC.

Signed-off-by: Aida Mynzhasova <aida.mynzhasova@skitlab.ru>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>