IIO: DAC: AD5791: Add support for the AD5760/AD5780 High Resolution DACs

Add support for the AD5760/AD5780 High Resolution Voltage Output DACs

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>