novena: Add MAINTAINERS file

Commit f91c09ac ("ARM: mx6: Add support for Kosagi Novena") missed to
add a MAINTAINERS file, so add Marek as the maintainer.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>