iio: imu: inv_mpu6050: disable i2c mux for MPU925x

Disable i2c mux for supported 9xxx chips. This is a
pre-requesite for controling 9xxx magnetometer using the
i2c master of the chip.

Check in device-tree that there is no i2c-gate device declared
for ensuring backward compatibility with existing setups.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>