iio: accell: mma8452: add vdd/vddio regulator operation support

The accelerometer's power supply could be controllable on some
platforms, such as i.MX6Q-SABRESD board, the mma8451's power supplies
are controlled by a GPIO fixed regulator, need to make sure the
regulators are enabled before any communication with mma8451, this
patch adds vdd/vddio regulator operation support.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>