dt-bindings: gpio: Add GPIO3 for AXP209 GPIO binding schema

The AXP209 has a 4th GPIO, so add it in preparation for support in the
driver.

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>