iio: st_accel: support the LIS331DL sensor

This adds support for the LIS331DL sensor version. This is
a simple 8bit-only accelerometer.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>