nios2: get cpu node with of_get_cpu_node

"device_type" use is deprecated for FDT though it has continued to be used
for nodes like cpu nodes. Use of_get_cpu_node() instead which works using
node names by default. This will allow the eventually removal of cpu
device_type properties.

Also, fix a leaked reference by adding a missing of_node_put.

Cc: Ley Foon Tan <lftan@altera.com>
Cc: nios2-dev@lists.rocketboards.org
Signed-off-by: Rob Herring <robh@kernel.org>