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 <[e-mail]>
Signed-off-by: Rafał Miłecki <[e-mail]>
Signed-off-by: Boris Brezillon <[e-mail]>

drivers/mtd/devices/powernv_flash.c

···
 	mtd->_read = powernv_flash_read;
 	mtd->_write = powernv_flash_write;
 	mtd->dev.parent = dev;
+	mtd_set_of_node(mtd, dev->of_node);
 	return 0;
 }