dt-bindings: iio: imu: mpu6050: Document invensense,mpu6880

MPU-6880 seems to be very similar to MPU-6500 / MPU-6050 and it works
fine with some minor additions for the mpu6050 driver.
Add a compatible for it to the binding documentation.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Cc: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Link: https://lore.kernel.org/r/20201202104656.5119-1-stephan@gerhold.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>