board: toradex: change maintainer to Francesco

Marcel is leaving Toradex and the email will start bouncing in a few
weeks, move maintainership to myself.

Cc: Marcel Ziswiler <marcel@ziswiler.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>