mtd: Add partition device node to mtd partition devices

The user visible change here is that mtd partitions get an of_node link
in sysfs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>