ASoC: dt-bindings: Document "samsung, arndale-wm1811" compatible

Add compatible string for boards with WM1811 CODEC to the list.
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Link: https://lore.kernel.org/r/20190920130218.32690-7-s.nawrocki@samsung.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/arndale.txt

 Audio Binding for Arndale boards
 
 Required properties:
-- compatible : Can be the following,
-			"samsung,arndale-rt5631"
+- compatible : Can be one of the following:
+		"samsung,arndale-rt5631",
+		"samsung,arndale-wm1811"
 
 - samsung,audio-cpu: The phandle of the Samsung I2S controller
 - samsung,audio-codec: The phandle of the audio codec