MAINTAINERS: Update NAND subsystem git repositories

NAND branches are now hosted on MTD repos, nand/next is on l2-mtd and
nand/fixes will be on linux-mtd.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[Brian: added branch names]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>