MAINTAINERS: add Freescale Vybrid SoC

Add Freescale Vybrid family as a own entry, along with an entry for
the so far orphan Vybrid device tree files. Also add myself as
a designated reviewer.

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>