[MTD] [NAND] Add kernel-doc for cellinfo field of struct nand_chip

Fixes kernel-doc warning in mtd/nand.h.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>