pinctrl: sunxi: Support A31/A31s with pinctrl variants

The A31s is a trimmed down version of the A31. Some hardware blocks
are removed, thus not available for muxing on the external pins.
Some external pins were directly removed.

This makes it easy to support the A31s pin controller with the A31
driver. We just mark the pins and functions that were trimmed as
A31 only.

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>