If a new cdev doesn't have a device_node defined when passed to
devfs_create(), set it to the device_node of the parent device, if one
exists.

For non-partitions, like ocotp or eeprom devices, this is the correct
thing to do.  Partitions need to use, and do use, a different node.

The code from commit 274e0b8 to set
device_node in ocotp can be removed.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>