pinctrl: sunxi: add driver for V3s SoC

V3s SoC features only a pin controller (for the lack of CPUs part).

Add a driver for this controller.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>