mtd: rawnand: Fix nand_chip kdoc

Describe the continuous read nand_chip fields to avoid the following
htmldocs warning:
include/linux/mtd/rawnand.h:1325: warning: Function parameter or member
'cont_read' not described in 'nand_chip'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 003fe4b9

 ("mtd: rawnand: Support for sequential cache reads")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230116094735.11483-1-miquel.raynal@bootlin.com