gpio: Add a reference to CEC on GPIO

This adds a pointer to the CEC GPIO driver from the GPIO list of
examples of drivers on top of GPIO.

Cc: linux-media@vger.kernel.org
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>