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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/dma/ti-edma.txt

···
 - ti,edma-reserved-slot-ranges: PaRAM slot ranges which should not be used by
 		the driver, they are allocated to be used by for example the
 		DSP. See example.
+- dma-channel-mask: Mask of usable channels.
+		Single uint32 for EDMA with 32 channels, array of two uint32 for
+		EDMA with 64 channels. See example and
+		Documentation/devicetree/bindings/dma/dma-common.yaml
+
 
 ------------------------------------------------------------------------------
 eDMA3 Transfer Controller
···
 	ti,edma-memcpy-channels = <20 21>;
 	/* The following PaRAM slots are reserved: 35-44 and 100-109 */
 	ti,edma-reserved-slot-ranges = <35 10>, <100 10>;
+	/* The following channels are reserved: 35-44 */
+	dma-channel-mask = <0xffffffff /* Channel 0-31 */
+			    0xffffe007>; /* Channel 32-63 */
 };
 
 edma_tptc0: tptc@49800000 {