s390: Use HAVE_MEMBLOCK_NODE_MAP

s390 used early_node_map[] just to prime free_area_init_nodes().  Now
memblock can be used for the same purpose and early_node_map[] is
scheduled to be dropped.  Use memblock instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux-s390@vger.kernel.org