iio: magnetometer: Add DT support for PNI RM3100

Signed-off-by: Song Qiang <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/magnetometer/pni,rm3100.txt

···
+* PNI RM3100 3-axis magnetometer sensor
+
+Required properties:
+
+- compatible : should be "pni,rm3100"
+- reg : the I2C address or SPI chip select number of the sensor.
+
+Optional properties:
+
+- interrupts: data ready (DRDY) from the chip.
+  The interrupts can be triggered on level high.
+
+Example:
+
+rm3100: rm3100@20 {
+	compatible = "pni,rm3100";
+	reg = <0x20>;
+	interrupt-parent = <&gpio0>;
+	interrupts = <4 IRQ_TYPE_LEVEL_HIGH>;
+};