dt-bindings: adi,adv7511.txt: document cec clock

Document the cec clock binding.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171007104658.14528-2-hverkuil@xs4all.nl