iio: magnetometer: add support to LIS2MDL

add support to STMicroelectronics LIS2MDL magnetometer in
st_magn framework

http://www.st.com/resource/en/datasheet/lis2mdl.pdf


Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>