iio: accel: add support to LIS2DE12

Add support to STMicroelectronics LIS2DE12 accelerometer in
st_accel framework.

http://www.st.com/resource/en/datasheet/lis2de12.pdf



Signed-off-by: Robert Jones <rjones@gateworks.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>