[media] s5p-cec.txt: document the HDMI controller phandle

Update the bindings documenting the new hdmi phandle.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
CC: linux-samsung-soc@vger.kernel.org
CC: devicetree@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>