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>