Blackfin: don't touch cpu_possible_map and cpu_present_map directly

We plan to remove cpu_possible_map and cpu_present_map later and we
have proper init_cpu_possible() and init_cpu_present() APIs.

Therefore this patch rewrites platform_init_cpus and platform_prepare_cpus
by their APIs.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>