pinctrl: sunxi: implement pin_config_set

The sunxi pinctrl only implements the pin_config_group_set callback at the
moment, whereas the gpiochip_generic_config function relies on
pin_config_set. Rework the functions a little to support pin_config_set,
and rely on it for pin_config_group_set.

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>