dt-bindings: gpio: Convert X-Powers AXP209 GPIO binding to a schema

The X-Powers AXP PMICs feature a GPIO Controller supported by Linux
thanks to its device tree binding.

Now that we have the DT validation in place, let's convert the device
tree bindings for that driver over to a YAML schema.

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210924071614.868307-1-maxime@cerno.tech