Staging: IIO: DAC: AD5624R: Update to IIO ABI

This driver did not conform with the IIO ABI for such devices.
Also the sysfs files that this driver adds were not complete and
partially un-documented.

Update and document ABI
Change License notice, stick to GPL-v2.
Fix indention style
Add option to specify external reference voltage via the regulator framework.
Add mandatory name attribute
Add mandatory out_scale attribute

Changes since V1:
Refine outY_powerdown_mode description
Remove bonus white line

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>