mtd: rawnand: Remove the cmx270 NAND controller driver

The CM-X270 board has been removed, we can remove the custom NAND
driver as well.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200518163300.304732-1-boris.brezillon@collabora.com