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.cpp

@@ -4,7 +4,7 @@
 #include "RAK4631Board.h"
 
 void RAK4631Board::begin() {
-  NRF52Board::begin();
+  NRF52BoardDCDC::begin();
   pinMode(PIN_VBAT_READ, INPUT);
 #ifdef PIN_USER_BTN
   pinMode(PIN_USER_BTN, INPUT_PULLUP);