pinctrl: sunxi: Remove redundant A31s pinctrl driver

Now that we can support the A31s pin controller with the A31 driver
using the new variants support, the independent A31s driver becomes
redundant.

Remove it.

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