mb/lenovo/t400: Correct display port list for [TW]500 variants

T500 and W500 (Coronado-5) use both digital display connectors. Both
with the DP AUX channel implemented, 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: I31e1415eff2d5d00c4a231906e3d861d2a59b629
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38573
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/mainboard/lenovo/t400/variants/t400/coronado-5/gma-mainboard.ads

@@ -23,6 +23,7 @@ private package GMA.Mainboard is
    ports : constant Port_List :=
      (DP1,
       HDMI1,
+      DP2,
       HDMI2,
       Analog,
       Internal,