AP_HAL: added disable_rxtx() to UARTs

libraries/AP_HAL/UARTDriver.h

@@ -142,4 +142,7 @@ public:
 
     // return true requested baud on USB port
     virtual uint32_t get_usb_baud(void) const { return 0; }
+
+    // disable TX/RX pins for unusued uart
+    virtual void disable_rxtx(void) const {}
 };