iio: magnetometer: mmc35240: Add DT binding doc

Signed-off-by: Jandy Gou <qingsong.gou@ck-telecom.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/magnetometer/mmc35240.txt

+* MEMSIC MMC35240 magnetometer sensor
+
+Required properties:
+
+  - compatible : should be "memsic,mmc35240"
+  - reg : the I2C address of the magnetometer
+
+Example:
+
+mmc35240@30 {
+        compatible = "memsic,mmc35240";
+        reg = <0x30>;
+};