pinctrl: sunxi: Rework the pin config building code

In order to support more easily the generic pinctrl properties, rework the
pinctrl maps configuration and split it into several sub-functions.

One of the side-effects from that rework is that we only parse the pin
configuration once, since it's going to be common to every pin, instead of
having to parsing once for each pin.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>