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>

board/kosagi/novena/MAINTAINERS

+NOVENA BOARD
+M:	Marek Vasut <marex@denx.de>
+S:	Maintained
+F:	board/kosagi/novena/
+F:	include/configs/novena.h
+F:	configs/novena_defconfig