ASoC: samsung: Add missing #sound-dai-cells property documentation

The #sound-dai-cells property might be helpful in selecting primary
or secondary CPU DAI and it's already present in i2s nodes for some
exynos SoCs so let's add it to the DT binding documentation.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/samsung-i2s.txt

@@ -60,6 +60,7 @@ Optional SoC Specific Properties:
   sub system(used in secondary sound source).
 - pinctrl-0: Should specify pin control groups used for this controller.
 - pinctrl-names: Should contain only one value - "default".
+- #sound-dai-cells: should be 1.
 
 
 Example:
@@ -79,4 +80,5 @@ i2s0: i2s@3830000 {
 	samsung,idma-addr = <0x03000000>;
 	pinctrl-names = "default";
 	pinctrl-0 = <&i2s0_bus>;
+	#sound-dai-cells = <1>;
 };