dt-bindings: remove deprecated compatible string from exynos-hdmi

samsung,exynos5-hdmi compatible was marked as deprecated in Jun 2013.
It was never used since then.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>