devicetree: Fix the dt_for_each_cpu_node

If the /cpus node contains nodes other than /cpus/cpu*, for example:
/cpus/cpu-map/. The test will issue an unexpected assert error as
follow:
[root@localhost]# ./arm-run arm/spinlock-test.flat
qemu-system-aarch64 -nodefaults -machine virt,gic-version=host,accel=kvm
 -cpu host -device virtio-serial-device -device virtconsole,chardev=ctd
-chardev testdev,id=ctd -device pci-testdev -display none -serial stdio
-kernel arm/spinlock-test.flat # -initrd /tmp/tmp.mwPLiF4EWm
lib/arm/setup.c:64: assert failed: ret == 0
        STACK:

In this patch, ignore the non-cpu subnodes instead of return an error.

Signed-off-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>