DEVELOPERS: add Lothar Felten as contact for BeagleBone defconfigs

Lothar added the beaglebone and beaglebone_qt5 defconfigs, and has
been regularly maintaining them, so it makes sense to have him listed
as a contact for those two defconfigs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Lothar Felten <lothar.felten@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>