LoongArch: Export some arch-specific pm interfaces

Some PMC (Power Management Controllers) need to support DTS and will use
the suspend interfaces thus this patch was to export such interfaces for
their use.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>