powerpc/85xx: Make kexec to interate over online cpus

This is not strictly required, because this iterates over logical
cpus and they are not (currently) discontigous. But, it's cleaner
code and more obvious what is going on

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>