Documentation: dt: iio: accel: add lng2dm sensor device binding

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

Documentation/devicetree/bindings/iio/st-sensors.txt

@@ -42,6 +42,7 @@ Accelerometers:
 - st,lsm303agr-accel
 - st,lis2dh12-accel
 - st,h3lis331dl-accel
+- st,lng2dm-accel
 
 Gyroscopes:
 - st,l3g4200d-gyro