soc: dt-bindings: add loongson-2 pm

Add the Loongson-2 SoC Power Management Controller binding with DT
schema format using json-schema.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230803063703.5659-2-zhuyinbo@loongson.cn


Signed-off-by: Arnd Bergmann <arnd@arndb.de>