Allow protected access and overriding methods

This allows extensions, e.g. a interrupt based timer.

hal_st/stm32fxxx/TimerStm.hpp

@@ -46,16 +46,17 @@ namespace hal
         TimerBaseStm(uint8_t aTimerIndex, Timing timing, const Config& config);
         ~TimerBaseStm();
 
-        void Start();
-        void Stop();
+        virtual void Start();
+        virtual void Stop();
+
+    protected:
+        TIM_HandleTypeDef handle = {};
 
     private:
         void ConfigureTrigger();
 
-    private:
         uint8_t timerIndex;
         Config config;
-        TIM_HandleTypeDef handle = {};
     };
 }