ASoC: dt-bindings: Document dl_mask property

SAI supports up to 8 data lines. This property let the user
configure how many data lines should be used per transfer
direction (Tx/Rx).

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>

Documentation/devicetree/bindings/sound/fsl-sai.txt

@@ -49,6 +49,11 @@ Optional properties:
 
   - big-endian		: Boolean property, required if all the SAI
 			  registers are big-endian rather than little-endian.
+  - fsl,dl_mask		: list of two integers (bitmask, first for RX, second
+			  for TX) representing enabled datalines. Bit 0
+			  represents first data line, bit 1 represents second
+			  data line and so on. Data line is enabled if
+			  corresponding bit is set to 1.
 
 Optional properties (for mx6ul):