pinctrl: sunxi: Handle bias disable

So far, putting NO_PULL in allwinner,pull was ignored, behaving like if
that property was not there at all.

Obviously, this is not the right thing to do, and in that case, we really
need to just disable the bias.

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