iio: magn: bmc150_magn: Add devicetree binding documentation

Add binding documentation for Bosch BMC150 magnetometer.
Signed-off-by: NIrina Tirdea <irina.tirdea@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/magnetometer/bmc150_magn.txt

+* Bosch BMC150 magnetometer sensor
+
+http://ae-bst.resource.bosch.com/media/products/dokumente/bmc150/BST-BMC150-DS000-04.pdf
+
+Required properties:
+
+  - compatible : should be "bosch,bmc150_magn"
+  - reg : the I2C address of the magnetometer
+
+Optional properties:
+
+  - interrupt-parent : phandle to the parent interrupt controller
+  - interrupts : interrupt mapping for GPIO IRQ
+
+Example:
+
+bmc150_magn@12 {
+        compatible = "bosch,bmc150_magn";
+        reg = <0x12>;
+        interrupt-parent = <&gpio1>;
+        interrupts = <0 1>;
+};