iio: magn: DT bindings for AK8974

This adds device tree bindings for the AK8974 magnetometer, similar
to those for the AK8975.

Cc: devicetree@vger.kernel.org
Cc: Samu Onkalo <samu.onkalo@intel.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/magnetometer/ak8974.txt

+* Asahi Kasei AK8974 magnetometer sensor
+
+Required properties:
+
+- compatible : should be "asahi-kasei,ak8974"
+- reg : the I2C address of the magnetometer
+
+Optional properties:
+
+- avdd-supply: regulator supply for the analog voltage
+  (see regulator/regulator.txt)
+- dvdd-supply: regulator supply for the digital voltage
+  (see regulator/regulator.txt)
+- interrupts: data ready (DRDY) and interrupt (INT1) lines
+  from the chip, the DRDY interrupt must be placed first.
+  The interrupts can be triggered on rising or falling
+  edges alike.
+- mount-matrix: an optional 3x3 mounting rotation matrix
+
+Example:
+
+ak8974@0f {
+	compatible = "asahi-kasei,ak8974";
+	reg = <0x0f>;
+	avdd-supply = <&foo_reg>;
+	dvdd-supply = <&bar_reg>;
+	interrupts = <0 IRQ_TYPE_EDGE_RISING>,
+		     <1 IRQ_TYPE_EDGE_RISING>;
+};