arm64: Use reboot_cpu instead of hardconding 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>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200323135110.30522-8-qais.yousef@arm.com