PSA: add safety mode

I'm in the process of reverse engineering the Vauxhall/Opel Corsa F, and
it looks as though the LKAS hardware is shared between
Citreon/Peugot/DS/Opel. Although PSA is now merged with Stellantis, I
think this is the best way to group these potential platforms in
openpilot.

car.capnp

@@ -603,6 +603,7 @@ struct CarParams {
     hyundaiCanfd @28;
     volkswagenMqbEvo @29;
     chryslerCusw @30;
+    psa @31;
   }
 
   enum SteerControlType {