AP_HAL: Added deinit() method to RCInput

Add a deinit() counterpart. This is needed for some ports that require some deinitializtion logic. The default implementation is empty. I'm not sure whether we need to inforce it for all.

libraries/AP_HAL/RCInput.h

@@ -16,6 +16,7 @@ public:
      * in the C++ type system.)
      */
     virtual void init(void* implspecific) = 0;
+    virtual void deinit() {};
 
     /**
      * Return true if there has been new input since the last read()