LoongArch: Let cores_io_master cover the largest NR_CPUS

Now loongson_system_configuration::cores_io_master only covers 64 cpus,
if NR_CPUS > 64 there will be memory corruption. So let cores_io_master
cover the largest NR_CPUS (256).

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