iio: light: add support for UVIS25 sensor

add support for STMicroelectronics UVIS25 uv sensor
http://www.st.com/resource/en/datasheet/uvis25.pdf

- continuos mode support
- i2c support
- spi support
- trigger mode support
- system PM support

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>