arm64: convert to arch_cpu_is_hotpluggable()

Convert arm64 to use the arch_cpu_is_hotpluggable() helper rather than
arch_register_cpu().

Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
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/E1r5R3g-00Cszg-PP@rmk-PC.armlinux.org.uk


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