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().

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Will Deacon <will@kernel.org>
CC: linux-arm-kernel@lists.infradead.org
CC: linux-kernel@vger.kernel.org