ARM: dtsi: axp81x: add GPIO DT node

This adds DT node for the GPIO/pinctrl part present in AXP813/AXP818.

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