iio: imu: inv_mpu6050: Separate driver into core and i2c functionality.

Separate this driver into core and i2c functionality.
This is in preparation for adding spi support.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>