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

include/linux/mtd/rawnand.h

@@ -1260,6 +1260,10 @@ struct nand_secure_region {
  * @read_retries: The number of read retry modes supported
  * @secure_regions: Structure containing the secure regions info
  * @nr_secure_regions: Number of secure regions
+ * @cont_read: Sequential page read internals
+ * @cont_read.ongoing: Whether a continuous read is ongoing or not
+ * @cont_read.first_page: Start of the continuous read operation
+ * @cont_read.last_page: End of the continuous read operation
  * @controller: The hardware controller	structure which is shared among multiple
  *              independent devices
  * @ecc: The ECC controller structure