gpio: axp209: Implement get_direction

Implement the get_direction callback for the GPIOs found in the AXP209
PMIC.

Due to the way they are implemented, in the same register you have the
muxing options, GPIO directions and GPIO values. Since you have no control
over what value is there at reset, simply use output as the default.

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