iio: imu: st_lsm6dsx: add support to lsm6ds3h

Add support to STM LSM6DS3H 6-axis (acc + gyro) Mems sensor

http://www.st.com/resource/en/datasheet/lsm6ds3h.pdf


Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>