riscv: convert to use arch_cpu_is_hotpluggable()

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

Acked-by: Palmer Dabbelt <palmer@rivosinc.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: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com> # On HiFive Unmatched
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/E1r5R4L-00Ct0d-To@rmk-PC.armlinux.org.uk


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