microblaze: 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: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>