Documentation: mtd: remove stale pxa3xx NAND controller documentation

It is preferred to have the documentation about the drivers directly
embedded in the driver itself. Remove this file now that the most
important information from this file have been re-written in
marvell_nand.c.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Ezequiel Garcia <ezequiel@collabora.com>