iio: magnetometer: Add DT support for PNI RM3100

* 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>;

};