media: docs: admin-guide: media: align HDMI CEC node names with dtschema

The bindings expect "cec" for HDMI CEC node.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>