arch: arm64: Fix coherence issue of SMP boot code

The current SMP boot code doesn't consider that the cores can boot at
the same time. Possibly, more than one core can boot into primary core
boot sequence. Fix it by using the atomic operation to make sure only
one core act as the primary core.

Correspondingly, sgi_raise_ipi should transfer CPU id to mpidr.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>