mtd: rawnand: oxnas: Keep track of registered devices

All initialized and registered devices should be listed somewhere so
that we can unregister/free them in the _remove() path.

This patch is not a fix per-se but is needed to apply three other
fixes coming right after, explaining the Fixes/Cc: stable tags.

Fixes: 66859249 ("mtd: nand: Add OX820 NAND Support")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200519130035.1883-36-miquel.raynal@bootlin.com