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 <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>