mtd: powernv_flash: set of_node in mtd's dev

This enables some features implemented in mtd subsystem like reading
label and partitioning info from DT.

Reported-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>