iio: imu: inv_mpu6050: add debugfs register r/w interface

The debugfs interface provides direct access to read and write device
registers if debugfs is enabled.

Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>