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>

MAINTAINERS

@@ -5898,6 +5898,7 @@ L:	linux-mtd@lists.infradead.org
 W:	http://www.linux-mtd.infradead.org/
 Q:	http://patchwork.ozlabs.org/project/linux-mtd/list/
 T:	git git://git.infradead.org/linux-mtd.git
+T:	git git://git.infradead.org/l2-mtd.git
 S:	Maintained
 F:	drivers/mtd/
 F:	include/linux/mtd/