dt-bindings: iio: add Bosch BMG160 gyroscope sensor

Define bindings for "bosch,bmg160" and "bosch,bmi055_gyro".

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>