RC_Output.h: add gpio methods

libraries/AP_HAL/RCOutput.h

@@ -336,6 +336,16 @@ public:
 
     virtual void timer_info(ExpandingString &str) {}
 
+    /*
+      Can this driver handle gpio as well as RC
+    */
+    virtual bool supports_gpio() { return false; };
+
+    /*
+      Writes gpio state to a channel
+    */
+    virtual void write_gpio(uint8_t chan, bool active) {};
+
     /*
      * calculate the prescaler required to achieve the desire bitrate
      */