dmaengine: pxa: document pxad_param

Add some documentation for the pxad_param structure, and describe the
contract behind the minimal required priority of a DMA channel.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Vinod Koul <vkoul@kernel.org>

include/linux/dma/pxa-dma.h

@@ -9,6 +9,15 @@ enum pxad_chan_prio {
 	PXAD_PRIO_LOWEST,
 };
 
+/**
+ * struct pxad_param - dma channel request parameters
+ * @drcmr: requestor line number
+ * @prio: minimal mandatory priority of the channel
+ *
+ * If a requested channel is granted, its priority will be at least @prio,
+ * ie. if PXAD_PRIO_LOW is required, the requested channel will be either
+ * PXAD_PRIO_LOW, PXAD_PRIO_NORMAL or PXAD_PRIO_HIGHEST.
+ */
 struct pxad_param {
 	unsigned int drcmr;
 	enum pxad_chan_prio prio;