mb/lenovo/t400: Correct display port list for [RT]400 variants

mb/lenovo/t400: Correct display port list for [RT]400 variants

The first digital display connector is unused, but strapped as if it
were on later revisions. The DP AUX channel of the second connector
is implemented, though, so add DP2 to the list.

Versions with a discrete GPU don't use external, digital connectors
but seem to have the straps correctly configured. So we hopefully
won't have to handle these specifically.

Based on schematics only, not tested.

Change-Id: I7d3e8b3a2123ddc407bb5a0cce86a3634b575f4a
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38572
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>