of: document refcount incrementation of of_get_cpu_node()

This function increments refcount.  This is worth noting.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/base.c

@@ -394,7 +394,8 @@ bool __weak arch_find_n_match_cpu_physical_id(struct device_node *cpun,
  * before booting secondary cores. This function uses arch_match_cpu_phys_id
  * which can be overridden by architecture specific implementation.
  *
- * Returns a node pointer for the logical cpu if found, else NULL.
+ * Returns a node pointer for the logical cpu with refcount incremented, use
+ * of_node_put() on it when done. Returns NULL if not found.
  */
 struct device_node *of_get_cpu_node(int cpu, unsigned int *thread)
 {