ARM: dts: exynos: Add #sound-dai-cells property to hdmi node in exynos5250.dtsi

This property is required for specifying link between the HDMI IP block
and the SoC's audio subsystem.

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