batman-adv: add debugfs structure for information per interface

To show information per interface, add a debugfs hardif structure
similar to the system in sysfs. Hard interface folders will be created
in "$debugfs/batman-adv/". Files are not yet added.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>