iio: light: isl29018: add vcc regulator operation support

The light sensor's power supply could be controllable by regulator
on some platforms, such as i.MX6Q-SABRESD board, the light sensor
isl29023's power supply is controlled by a GPIO fixed regulator,
need to make sure the regulator is enabled before any operation of
sensor, this patch adds vcc regulator operation support.

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