pinctrl: move gpio-axp209 to pinctrl

To prepare the driver for the upcoming pinctrl features, move the GPIO
driver AXP209 from GPIO to pinctrl subsystem.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>