ASoC: arizona: Add device tree binding doc for volume limits

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/wlf,arizona.txt

@@ -47,3 +47,7 @@ Optional properties:
     See the datasheet for values.
     The second cell is ignored for codecs that do not have OUT6 (wm5102, wm8997,
     wm8998, wm1814)
+
+  - wlf,out-volume-limit : The volume limit value that should be applied to each
+    output channel. See the datasheet for exact values. Channels are specified
+    in the order OUT1L, OUT1R, OUT2L, OUT2R, etc.