variants: XIAO NRF52: Enable DC/DC regulator

This reduces the power consumption by approximately 25%.

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

variants/xiao_nrf52/XiaoNrf52Board.cpp

@@ -23,6 +23,7 @@ static void disconnect_callback(uint16_t conn_handle, uint8_t reason) {
 void XiaoNrf52Board::begin() {
   // for future use, sub-classes SHOULD call this from their begin()
   startup_reason = BD_STARTUP_NORMAL;
+  NRF_POWER->DCDCEN = 1;
 
   pinMode(PIN_VBAT, INPUT);
   pinMode(VBAT_ENABLE, OUTPUT);