pinctrl: sunxi: Remove PINCTRL_SUNXI

The PINCTRL_SUNXI configuration was kept only to deal with the introduction of
per-machine symbols and the various pintrl drivers through different tree.

Now that it's not useful anymore, we can just remove it.

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