of: Add cpu node iterator for_each_of_cpu_node()

of: Add cpu node iterator for_each_of_cpu_node()

Iterating thru cpu nodes is a common pattern. Create a common iterator
which can find child nodes either by node name or device_type == cpu.
Using the former will allow for eventually dropping device_type
properties which are deprecated for FDT.

Cc: Frank Rowand <[email protected]>
Reviewed-by: Geert Uytterhoeven <[email protected]>
Signed-off-by: Rob Herring <[email protected]>