pinctrl: sunxi: Declare set_config on the GPIO chip

Our pin controller can configure the pins no matter how they are muxed, so
it makes sense to allow this for GPIOs as well.

Add the generic set_config function so that we can rely on the existing
pinctrl code we have.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>