pinctrl: sunxi: Add common sun5i pinctrl driver

The sun5i SoCs (A10s, A13, GR8) are all based on the same die fit in
different packages. Hence, the pins and functions available are just the
based on the same set, each SoC having a different subset.

Introduce a common pinctrl driver that supports multiple variants to allow
to put as much as we can in common.

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