Deduplicate reboot() for NRF52 boards

The reboot() method is the same for all NRF52 boards. Use a shared
implementation.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

variants/promicro/PromicroBoard.h

@@ -75,10 +75,6 @@ public:
       return 0;
   }
 
-  void reboot() override {
-    NVIC_SystemReset();
-  }
-  
   void powerOff() override {
     sd_power_system_off();
   }