mtd: nand: Add a NAND page I/O request type

Use an enum to differentiate the type of I/O (reading or writing a
page). Also update the request iterator.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-5-miquel.raynal@bootlin.com