dt-bindings: exynos: Document #sound-dai-cells property of the HDMI node

The #sound-dai-cells DT property is required to describe link between
the HDMI IP block and the SoC's audio subsystem.
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

Documentation/devicetree/bindings/display/exynos/exynos_hdmi.txt

@@ -16,6 +16,7 @@ Required properties:
 - ddc: phandle to the hdmi ddc node
 - phy: phandle to the hdmi phy node
 - samsung,syscon-phandle: phandle for system controller node for PMU.
+- #sound-dai-cells: should be 0.
 
 Required properties for Exynos 4210, 4212, 5420 and 5433:
 - clocks: list of clock IDs from SoC clock driver.