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

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.

This is a port of linux commit
701981cab016 ("mtd: nand: Add a NAND page I/O request type")

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
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu> # U-Boot port
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>