LoongArch: convert to use arch_cpu_is_hotpluggable()

Convert loongarch to use the arch_cpu_is_hotpluggable() helper rather
than arch_register_cpu(). Also remove the export as nothing should be
using arch_register_cpu() outside of the core kernel/acpi code.

Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/E1r5R4B-00Ct0G-Kk@rmk-PC.armlinux.org.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>