iio: imu: inv_mpu6050: rewrite power and engine management

Rewrite clock management to use automatic clock switching
present since MPU6500.
Sensors engine management can now turn on or off a batch of
sensors which simplifies usage a lot.
Temperature sensor is now turned on/off depending on usage.

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