mtd: rawnand: sunxi: Document 'sunxi_nfc's 'caps' member

Fixes the following W=1 kernel build warning(s):

 drivers/mtd/nand/raw/sunxi_nand.c:250: warning: Function parameter or member 'caps' not described in 'sunxi_nfc'

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Boris BREZILLON <b.brezillon.dev@gmail.com>
Cc: Qiang Yu <yuq825@gmail.com>
Cc: Dmitriy B <rzk333@gmail.com>
Cc: Sergey Lapin <slapin@ossfans.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201109182206.3037326-21-lee.jones@linaro.org

drivers/mtd/nand/raw/sunxi_nand.c

@@ -233,6 +233,7 @@ struct sunxi_nfc_caps {
  *	   controller
  * @complete: a completion object used to wait for NAND controller events
  * @dmac: the DMA channel attached to the NAND controller
+ * @caps: NAND Controller capabilities
  */
 struct sunxi_nfc {
 	struct nand_controller controller;