csky: Don't leak device tree node reference

of_find_node_by_type() acquires a reference to the node returned by it
and that reference needs to be dropped by its caller. setup_smp()
doesn't do that, so fix it by converting to for_each_of_cpu_node().

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Guo Ren <guoren@kernel.org>