Doxygen: exclude drivers/libtsm

drivers/libtsm includes a bunch of Doxygen comments, and worst of all, a
"@mainpage" tag controlling Doxygen's main page. Since libtsm isn't really
part of the API we intend to advertise, let's exclude this directory from
being scanned by doxygen.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>