MAINTAINERS: Move all MTD related branches to a single repo

Historically, branches targeting the next release (and pulled in
linux-next) have been pushed on the l2-mtd repo and fixes branches on
the linux-mtd one. Now that all MTD maintainers have RW permissions on
linux-mtd tree, there's no good reason to have two different trees.
Move all -next branches to linux-mtd.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Cc: Richard Weinberger <richard@nod.at>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Acked-by: Richard Weinberger <richard@nod.at>
Acked-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>