dmaengine: mmp_pdma: add missed of_dma_controller_free

The driver calls of_dma_controller_register in probe but does not free
it in remove.
Add the call to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Link: https://lore.kernel.org/r/20191115083153.12334-1-hslester96@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>