mtd: nand: add myself as NAND co-maintainer

I have been actively reviewing and contributing improvements to the NAND
subsystem for more than 6 months now and am willing to continue doing so
in the future. Formalize my new role.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>