LoongArch: Enable PG when wakeup from suspend

Some firmwares don't enable PG when wakeup from suspend, so do it in
kernel. This can improve code compatibility for boot kernel.

Signed-off-by: Baoqi Zhang <zhangbaoqi@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>