dt-bindings: pinctrl: add loongson-2 pinctrl

Add the Loongson-2 pinctrl binding with DT schema format using
json-schema.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Link: https://lore.kernel.org/r/20221114024942.8111-2-zhuyinbo@loongson.cn
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>