axxia: add sync between cpu_die and cpu_kill

Normally, a CPU should run cpu_die first and then the CPUs stand by it
are in charge of killing it by calling cpu_kill. If a CPU doesn't call
cpu_die before other CPUs call cpu_kill, the system will hang. So add
synchronization implement between cpu_die and cpu_kill.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>