Maintainers.txt: update mailing list information
    
    The devel mailing list has moved from 01.org to groups.io - update
    the mailing list information to reflect this.
    
    Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>