MAINTAINERS: add more people to the MTD maintainer team

Brian has been maintaining the MTD subsystem alone for several years
now, and maintaining such a subsystem can really be time consuming.

Create a maintainer team formed of the most active MTD contributors
to help Brian with this task, which will hopefully improve the
subsystem reactivity.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>