of: move kobj_to_device_node() into dynamic.c

The only user of kobj_to_device_node() is in dynamic.c, so move it
there. This avoids having to make it conditional once kobject is
configurable.

Tested-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/dynamic.c

@@ -16,6 +16,11 @@
 
 #include "of_private.h"
 
+static struct device_node *kobj_to_device_node(struct kobject *kobj)
+{
+	return container_of(kobj, struct device_node, kobj);
+}
+
 /**
  * of_node_get() - Increment refcount of a node
  * @node:	Node to inc refcount, NULL is supported to simplify writing of