pinctrl: sunxi: Support generic binding

Our bindings are mostly irrelevant now that we have generic pinctrl
bindings that cover exactly the same uses cases.

Add support for the new ones, and obviously keep our old binding support in
order to keep the ABI stable.

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>