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/rak4631/variant.h

@@ -104,6 +104,14 @@ extern "C"
 	static const uint8_t A7 = PIN_A7;
 #define ADC_RESOLUTION 14
 
+// Power management boot protection threshold (millivolts)
+// Set to 0 to disable boot protection
+#define PWRMGT_VOLTAGE_BOOTLOCK 3300   // Won't boot below this voltage (mV)
+// LPCOMP wake configuration (voltage recovery from SYSTEMOFF)
+// AIN3 = P0.05 = PIN_A0 / PIN_VBAT_READ
+#define PWRMGT_LPCOMP_AIN 3
+#define PWRMGT_LPCOMP_REFSEL 4  // 5/8 VDD (~3.13-3.44V)
+
 // Other pins
 #define PIN_AREF (2)
 #define PIN_NFC1 (9)