MAINTAINERS: Add myself as MTD maintainer

Help Richard and hopefully Tudor and Vignesh maintaining MTD since
Boris is leaving.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Richard Weinberger <richard@nod.at>