dma: mmp_pdma: set DMA_PRIVATE

As the driver now has its own xlate function and makes use of the
dma_get_slave_channel(), we need to manually set the DMA_PRIVATE flags.

Drivers which rely on of_dma_simple_xlate() do implicitly the same by
going through __dma_request_channel().

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>