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

drivers/dma/at_xdmac.c

@@ -1394,6 +1394,11 @@ static int at_xdmac_probe(struct platform_device *pdev)
 	dma_cap_set(DMA_CYCLIC, atxdmac->dma.cap_mask);
 	dma_cap_set(DMA_MEMCPY, atxdmac->dma.cap_mask);
 	dma_cap_set(DMA_SLAVE, atxdmac->dma.cap_mask);
+	/*
+	 * Without DMA_PRIVATE the driver is not able to allocate more than
+	 * one channel, second allocation fails in private_candidate.
+	 */
+	dma_cap_set(DMA_PRIVATE, atxdmac->dma.cap_mask);
 	atxdmac->dma.dev				= &pdev->dev;
 	atxdmac->dma.device_alloc_chan_resources	= at_xdmac_alloc_chan_resources;
 	atxdmac->dma.device_free_chan_resources		= at_xdmac_free_chan_resources;