dt-bindings: iio: magn: add LSM9DS1 bindings

Add DT binding documentation for the LSM9DS1 magnetometer.
Signed-off-by: Martin Kelly <martin@martingkelly.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -67,6 +67,7 @@ Magnetometers:
 - st,lsm303dlm-magn
 - st,lis3mdl-magn
 - st,lis2mdl
+- st,lsm9ds1-magn
 
 Pressure sensors:
 - st,lps001wp-press