variants: RAK4631: Enable DC/DC regulator to reduce power consumption

The RAK4631/RAK4630 module are able to use the DC/DC converter. Enable
it to reduce power consumption.

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

variants/rak4631/RAK4631Board.h

@@ -29,7 +29,7 @@
 #define  PIN_VBAT_READ    5
 #define  ADC_MULTIPLIER   (3 * 1.73 * 1.187 * 1000)
 
-class RAK4631Board : public NRF52BoardOTA {
+class RAK4631Board : public NRF52BoardDCDC, public NRF52BoardOTA {
 public:
   RAK4631Board() : NRF52BoardOTA("RAK4631_OTA") {}
   void begin();