staging: iio: ad5933: change regulator binging for vref

Set a single voltage regulator for all voltage references.
Remove voltage reference value from default platafrom data struct.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Gabriel Capella <gabriel@capella.pro>
Co-developed-by: Gabriel Capella <gabriel@capella.pro>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>