MAINTAINERS: Add myself as co-maintainer for MTD

I have been contributing and reviewing MTD patches for a while now and
would like to add myself as co-maintainer for MTD to help maintain
CFI and SPI NOR parts of the subsystem.
Hopefully, this will help in taking some load off current maintainers.

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