staging: iio: ad7192: add DVdd regulator

The AD7190/AD7192/AD7193/AD7195 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>