[media] bindings for stih-cec driver

Add bindings documentation for stih-cec driver.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>