iio: imu: st_lsm6dsx: add support to ism330dlc

Add support to STM ISM330DLC 6-axis (acc + gyro) Mems sensor
http://www.st.com/resource/en/datasheet/ism330dlc.pdf



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