MAINTAINERS: MTD Git repository is hosted on kernel.org

Because of the slowness of the infradead.org server, Linus requested
that we send him PRs from another domain. We moved our Git repository
to kernel.org and maintain the following branches:
* Fixes based on the last -rc: mtd/fixes
* Next branch based usually on -rc1: mtd/next, nand/next, spi-nor/next.

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