of_path: Drop possible further extensions of device-path property
Originally it was intended to further extend the multi string property
device-path further with more elements, like for example a filename. It
turned out though that this is too complex and instead of further
extending the property we should instead create additional properties,
so this mechanism is removed with this patch to make the code simpler.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>