arm64: smp: Jump back to secondary start kernel for onlined CPUs

Add a call to secondary start kernel for cpus which have been
onlined via the hotplug path for wfi based hotplug solution
where the return path from cpu_die should not return to the
idle thread. Update the cpu_die definition with a __ref to
allow referencing a __cpuinit call (secondary_start_kernel)
from it.

Change-Id: I7c083effda3928b562ea0d601833ceb8d5178d43
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>