media: dt-bindings: document the tegra CEC bindings

This documents the binding for the Tegra CEC module.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>