of/platform: clarify of_find_device_by_node refcounting

Add comment clarifying that of_find_device_by_node() takes a reference
to the embedded struct device which needs to be dropped after use.

Note that most current users fail to do so.

Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

drivers/of/platform.c

···
  * of_find_device_by_node - Find the platform_device associated with a node
  * @np: Pointer to device tree node
  *
+ * Takes a reference to the embedded struct device which needs to be dropped
+ * after use.
+ *
  * Returns platform_device pointer, or NULL if not found
  */
 struct platform_device *of_find_device_by_node(struct device_node *np)