MAINTAINERS: Update SPI NOR subsystem git repositories

SPI NOR branches are now hosted on MTD repos, spi-nor/next is on l2-mtd
and spi-nor/fixes is on linux-mtd.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>