csky: use free_initmem_default() in free_initmem()

The existing code is essentially
free_initmem_default()->free_reserved_area() without poisoning.

Note that existing code missed to update the managed page count of the
zone.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Tested-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Guo Ren <guoren@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>