ARM: dts: exynos: Add #sound-dai-cells property to exynos5250 i2s nodes

The #sound-dai-cells property may be required to reference the CPU DAI
properly. This change is required for Snow HDMI audio.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>