mtd: rawnand: atmel: Fix error handling path in atmel_nand_controller_add_nands

mtd: rawnand: atmel: Fix error handling path in atmel_nand_controller_add_nands

In case of a jump to the  err label due to atmel_nand_create() or
atmel_nand_controller_add_nand() failure, the reference to nand_np
need to be released

Use for_each_child_of_node_scoped() to fix the issue.

Fixes: f88fc122cc34 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Signed-off-by: Erick Karanja <karanja99erick@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>