dt-bindings: dma: ti-edma: Document dma-channel-mask for EDMA

Similarly to paRAM slots, channels can be used by other cores.

The common dma-channel-mask property can be used for specifying the
available channels.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191025073056.25450-3-peter.ujfalusi@ti.com


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