MAINTAINERS: add myself as SPI NOR co-maintainer

I have been reviewing and contributing to the SPI NOR subsystem
for the last few months and I'm willing to continue doing so.
Volunteer as a maintainer for the SPI NOR part of the MTD subsystem.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>