mtd: rawnand: ams-delta: Use private structure

Introduce a driver private structure and allocate it on device probe.
Use it for storing nand_chip structure, GPIO descriptors prevoiusly
stored in static variables as well as io_base pointer previously passed
as nand controller data or platform driver data.  Subsequent patches
may populate the structure with more members as needed.

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