dmaengine: at_xdmac: Add DMA_PRIVATE

same issue as commit 7f5ae355:
"Without DMA_PRIVATE the driver is not able to allocate more than one channel.
Since it uses dma_get_any_slave_channel that calls private_candidate, the
second allocation fails at
/* some channels are already publicly allocated */
"

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>