pinctrl: sunxi: Add pinctrl variants

Some SoCs are either supposed to be pin compatible (A10 and A20 for
example), or are just repackaged versions of the same die (A10s, A13, GR8).

In those case, having a full blown pinctrl driver just introduces
duplication in both data size and maintainance effort.

Add a variant option to both pins and functions to be able to limit the
pins and functions described only to a subset of the SoC we support with a
given driver.

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