dt-bindings: iio: imu: Add inv_icm42600 documentation

Document the ICM-426xxx devices devicetree bindings.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>