dt-bindings: pinctrl: meson: add output support in pinconf

add support for the pinconf DT property output-enable, output-disable,
output-low and output-high in the meson pinctrl driver.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>