mtd: rawnand: rockchip: Add missing title to a kernel doc comment

All fields of the nfc_cfg structure are documented but the name, which
leads to a W=1 warning. Add a title.

Fixes: 058e0e84

 ("mtd: rawnand: rockchip: NFC driver for RK3308, RK2928 and others")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231211150704.109138-2-miquel.raynal@bootlin.com