staging: iio: ad9832: add DVDD regulator

The AD9832/AD9835 is supplied with two power sources: AVDD as analog
supply voltage and DVDD as digital supply voltage.

Attempt to fetch and enable the regulator 'dvdd'. Bail out if any error
occurs.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>