iio: light: add driver for veml6030 ambient light sensor

veml6030 is an ambient light sensor from Vishay semiconductors.
It has 16-bit resolution, supports both ambient light measurement
and white channel which is more responsive to wider wavelength
spectrum. It has flexible power saving, integration time and
gain options. Communication with host is over I2C.

Signed-off-by: Rishi Gupta <gupt21@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>