staging:iio:cdc:ad7150: Really basic regulator support.

Given DT docs will include regulators, lets just turn them on and
off with driver probe() and remove().

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-20-jic23@kernel.org