mtd: rawnand: Create a nand_chip operations structure

And move nand_chip hooks there.

While moving entries from one structure to the other, adapt the
documentation style.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-4-miquel.raynal@bootlin.com