iio: st_sensors: add device tree bindings

This adds some basic, simple device tree bindings to the STMicro
MEMS sensor drivers.

Cc: devicetree@vger.kernel.org
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Denis CIOCCA <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>