iio: accel: bma400: basic regulator support

Add support for the VDD and VDDIO regulators using the regulator
framework.

Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>