MAINTAINERS: Add myself as co-maintainer of the Arasan NAND controller driver

When I submitted the driver I added Naga as Maintainer and forgot to
add myself.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210505213750.257417-20-miquel.raynal@bootlin.com