mtd: rawnand: arasan: Document 'anfc_op's 'buf' member

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

 drivers/mtd/nand/raw/arasan-nand-controller.c:133: warning: Function parameter or member 'buf' not described in 'anfc_op'

Cc: Naga Sureshkumar Relli <nagasure@xilinx.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Choudary Kalluri <punnaia@xilinx.com>
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/20201109182206.3037326-22-lee.jones@linaro.org