dt-bindings: iio: gyro: Add DT binding doc for ADXRS290

Add devicetree binding document for ADXRS290, a dual-axis MEMS gyroscope.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nishant Malpani <nish.malpani25@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>