[media] dt-bindings: media/s5p-cec.txt: document needs-hpd property

Needed for boards that wire the CEC pin in such a way that it
is unavailable when the HPD is low.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>