nRF52840 Power Management - Phase 1 - Boot Low VBAT Voltage Lockout

Added NRF52840 power management core functionality:
- Boot‑voltage lockout
- Initial support for shutdown/reset reason storage and capture (via RESETREAS/GPREGRET2)
- LPCOMP wake (for voltage-driven shutdowns)
- VBUS wake (for voltage-driven shutdowns)
- Per-board shutdown handler for board-specific tasks
- Exposed CLI queries for power‑management status in CommonCLI.cpp
- Added documentation in docs/nrf52_power_management.md.
- Enabled power management support in Xiao nRF52840, RAK4631, Heltec T114 boards

variants/xiao_nrf52/XiaoNrf52Board.h

@@ -6,7 +6,12 @@
 
 #ifdef XIAO_NRF52
 
-class XiaoNrf52Board : public NRF52BoardDCDC, public NRF52BoardOTA {
+class XiaoNrf52Board : public NRF52BoardDCDC, public NRF52BoardOTA { 
+protected:
+#if NRF52_POWER_MANAGEMENT
+  void initiateShutdown(uint8_t reason) override;
+#endif
+
 public:
   XiaoNrf52Board() : NRF52BoardOTA("XIAO_NRF52_OTA") {}
   void begin();
@@ -20,21 +25,7 @@ public:
   }
 #endif
 
-  uint16_t getBattMilliVolts() override {
-    // Please read befor going further ;)
-    // https://wiki.seeedstudio.com/XIAO_BLE#q3-what-are-the-considerations-when-using-xiao-nrf52840-sense-for-battery-charging
-
-    // We can't drive VBAT_ENABLE to HIGH as long
-    // as we don't know wether we are charging or not ...
-    // this is a 3mA loss (4/1500)
-    digitalWrite(VBAT_ENABLE, LOW);
-    int adcvalue = 0;
-    analogReadResolution(12);
-    analogReference(AR_INTERNAL_3_0);
-    delay(10);
-    adcvalue = analogRead(PIN_VBAT);
-    return (adcvalue * ADC_MULTIPLIER * AREF_VOLTAGE) / 4.096;
-  }
+  uint16_t getBattMilliVolts() override;
 
   const char* getManufacturerName() const override {
     return "Seeed Xiao-nrf52";