LoongArch: Do not create sysfs control file for io master CPUs

Now io master CPUs are not hotpluggable on LoongArch, but in the current
code only /sys/devices/system/cpu/cpu0/online is not created. Let us set
the hotpluggable field of all the io master CPUs as 0, then prevent to
create sysfs control file for all the io master CPUs which confuses some
user space tools. This is similar with commit 9cce844a

 ("MIPS: CPU#0
is not hotpluggable").

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