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>