mtd: rawnand: ams-delta: Explicitly inherit from nand_controller

All NAND objects are supposed to inherit from nand_controller. The
framework is providing a dummy controller object, but we're moving
away from this approach in favor of explicit inheritance.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>