pinctrl: meson: meson8: add the AO HDMI CEC pin

This adds another missing pin found in the Meson8 SoCs. Currently
there's no driver which would use this pin yet.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>