powerpc/85xx: Minor fixups for kexec on 85xx

Make kexec_down_cpus atmoic since it will be incremented by all cores as
they are coming down.

Remove duplicate calls to mpc85xx_smp_kexec_down, now it's called by the
crash and normal kexec pathway only once.

Increase the timeout to wait for other cores to shutdown.

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