mfd: axp20x: Add AXP209 GPIO support

Now that we have a GPIO driver for the AXP209, we can add it to our MFD.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>