iio: imu: inv_mpu6050: add defines for supporting 9-axis chips

Add registers defines required for driving chip i2c master ip.
Add MPU9xxx magnetometer scan elements and update data bytes size.

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