iio: magnetometer: mag3110: add vdd/vddio regulator operation support

The magnetometer's power supplies could be controllable on some platforms,
such as i.MX6Q-SABRESD board, the mag3110's power supplies are controlled
by a GPIO fixed regulator, need to make sure the regulators are enabled
before any communication with mag3110, this patch adds vdd/vddio regulator
operation support.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>