pinctrl: pinctrl-loongson2: add pinctrl driver support

The Loongson-2 SoC has a few pins that can be used as GPIOs or take
multiple other functions. Add a driver for the pinmuxing.

There is currently no support for GPIO pin pull-up and pull-down.

Signed-off-by: zhanghongchen <zhanghongchen@loongson.cn>
Co-developed-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Link: https://lore.kernel.org/r/20221114024942.8111-1-zhuyinbo@loongson.cn


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>