of: use for_each_of_cpu_node iterator

Use the for_each_of_cpu_node iterator to iterate over cpu nodes. This
has the side effect of defaulting to iterating using "cpu" node names in
preference to the deprecated (for FDT) device_type == "cpu".

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>