iio: gyro: add DT bindings to fxas21002c

Add device tree bindings for the FXAS21002C gyroscope.
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/gyroscope/nxp,fxas21002c.txt

+* NXP FXAS21002C Gyroscope device tree bindings
+
+http://www.nxp.com/products/sensors/gyroscopes/3-axis-digital-gyroscope:FXAS21002C
+
+Required properties:
+  - compatible : should be "nxp,fxas21002c"
+  - reg : the I2C address of the sensor or SPI chip select number for the
+          device.
+  - vdd-supply: phandle to the regulator that provides power to the sensor.
+  - vddio-supply: phandle to the regulator that provides power to the bus.
+
+Optional properties:
+  - reset-gpios : gpio used to reset the device, see gpio/gpio.txt
+  - interrupts : device support 2 interrupts, INT1 and INT2,
+                 the interrupts can be triggered on rising or falling edges.
+                 See interrupt-controller/interrupts.txt
+  - interrupt-names: should contain "INT1" or "INT2", the gyroscope interrupt
+                     line in use.
+  - drive-open-drain: the interrupt/data ready line will be configured
+                      as open drain, which is useful if several sensors share
+                      the same interrupt line. This is a boolean property.
+                      (This binding is taken from pinctrl/pinctrl-bindings.txt)
+
+Example:
+
+gyroscope@20 {
+	compatible = "nxp,fxas21002c";
+	reg = <0x20>;
+	vdd-supply = <&reg_peri_3p15v>;
+	vddio-supply = <&reg_peri_3p15v>;
+};