c6x: 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: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <jacquiot.aurelien@gmail.com>
Cc: linux-c6x-dev@linux-c6x.org
Signed-off-by: Rob Herring <robh@kernel.org>