media: dt-bindings: document the CEC GPIO bindings

Document the bindings for the cec-gpio module for hardware where the
CEC line and optionally the HPD line are connected to GPIO lines.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>