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>

Documentation/driver-api/gpio/drivers-on-gpio.rst

@@ -85,6 +85,10 @@ hardware descriptions such as device tree or ACPI:
   any other serio bus to the system and makes it possible to connect drivers
   for e.g. keyboards and other PS/2 protocol based devices.
 
+- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
+  Consumer Electronics Control bus using only GPIO. It is used to communicate
+  with devices on the HDMI bus.
+
 Apart from this there are special GPIO drivers in subsystems like MMC/SD to
 read card detect and write protect GPIO lines, and in the TTY serial subsystem
 to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The