ia64: Replace cpu_down() with smp_shutdown_nonboot_cpus()

Use the new smp_shutdown_nonboot_cpus() instead of using cpu_down()
directly.

Use reboot_cpu instead of hardcoding the boot CPU to 0.

This also prepares to make cpu_up/down() a private interface of the CPU
subsystem.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: https://lkml.kernel.org/r/20200323135110.30522-4-qais.yousef@arm.com