iio: imu: inv_mpu6050: inform the i2c mux core about how it is used

The i2c mux core can then take appropriate action depending on if it is
used for an actual i2c mux, or for an arbitrator or gate. In this case
it is used as a gate.

This will make devicetree bindings simpler when they are eventually
added.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>