iio: st-accel: add support for lis2dh12

iio: st-accel: add support for lis2dh12

This commit add support for STMicroelectronics lis2dh12 accelerometer.
Datasheet for this device can be found here:

http://www.st.com/st-web-ui/static/active/en/resource/technical/
document/datasheet/DM00091513.pdf

Signed-off-by: Giuseppe Barba <giuseppe.barba@st.com>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>