riscv: use for_each_of_cpu_node iterator

Use the new for_each_of_cpu_node() helper to iterate over cpu nodes
instead of open coding. Note that this will allow matching also on the
node name instead of the (for FDT) deprecated device_type property.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>