powerpc/4xx: Fix regression in SMP on 476

commit c56e5853 breaks SMP support in PPC_47x chip.
 secondary_ti must be set to current thread info before callin kick_cpu or else
 start_secondary_47x will jump into void when trying to return to c-code.
 In the current setup secondary_ti is initialized before the CPU idle task is started
 and only the boot core will start. I am not sure this is the correct solution, but it
 makes SMP possible in my chip.
 Note! The HOTPLUG support probably need some fixing to, There is no trampoline code
 available in head_44x.S - start_secondary_resume?

Signed-off-by: Kerstin Jonsson <kerstin.jonsson@ericsson.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>