mtd: rawnand: Reorganize the nand_chip structure

Reorder fields in this structure and pack entries by theme:
* The main descriptive structures
* The data interface details
* Bad block information
* The device layout
* Extra buffers matching the device layout
* Internal values
* External objects like the ECC controller, the ECC engine and a
  private data pointer.

While at it, adapt the documentation style.

I changed on purpose the description of @oob_poi which was weird.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-7-miquel.raynal@bootlin.com