Rover: add Pixhawk pin to RSSI_PIN's @Value

Also prepended APM2 to existing values.  Added A2 to list to be
consistent with plane and copter

APMrover2/Parameters.pde

@@ -38,7 +38,7 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @Param: RSSI_PIN
     // @DisplayName: Receiver RSSI sensing pin
     // @Description: This selects an analog pin for the receiver RSSI voltage. It assumes the voltage is 5V for max rssi, 0V for minimum
-    // @Values: -1:Disabled, 0:A0, 1:A1, 13:A13
+    // @Values: -1:Disabled, 0:APM2 A0, 1:APM2 A1, 2:APM2 A2, 13:APM2 A13, 103:Pixhawk SBUS
     // @User: Standard
     GSCALAR(rssi_pin,            "RSSI_PIN",         -1),