MAINTAINERS: add l2-mtd.git, 'next' tree for MTD

We've been semi-officially queueing patches here for a while, and it's
in linux-next, so let's advertise it in MAINTAINERS.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>