pinctrl: sunxi: Deal with configless pins

Even though the our binding had the assumption that the allwinner,pull and
allwinner,drive properties were optional, the code never took that into
account.

Fix that.

Signed-off-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>