staging:iio:ad799x use regulator for vref

Switch the ad799x driver to use the regulator framework and add binding for reference voltage.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>