[media] dt-bindings: media/s5p-cec.txt, media/stih-cec.txt: refer to cec.txt

Now that there is a cec.txt with common CEC bindings, update the two
driver-specific bindings to refer to cec.txt.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>