MAINTAINERS: Add BATMAN ADVANCED documentation files

The sysfs ABI documentation files and the batman-adv.txt are maintained by
the BATMAN ADVANCED maintainers and patches for them should therefore be
sent to them.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>