Add a device tree property description for hdmi device node
. '#sound-dai-cells' property is required to describe link between
the HDMI IP block and the SoC's audio subsystem and Exynos SoC device
tree files already have this property but we missed its description.
* tag 'exynos-drm-fixes-for-v4.16-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/daeinki/drm-exynos:
dt-bindings: exynos: Document #sound-dai-cells property of the HDMI node