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>