ARM: Use reboot_cpu instead of hardcoding it to 0

Use `reboot_cpu` variable instead of hardcoding 0 as the reboot cpu in
machine_shutdown().

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://lkml.kernel.org/r/20200323135110.30522-6-qais.yousef@arm.com