Allow deepsleep in rak4630 and make it restart well when power comes back (#7882)

* Make RAK4631 nodes power back on deep sleep

The devices will hang if the VBAT goes under 1.7V (Brown-out reset) and
they will never come back unless power supply goes completely off.

This kills unattended nodes.

Using the SystemOff the LPCOMP we can get the nodes back again when
power comes back, even if VBAT goes under 1.7V, which moreover is more
unlikely because the device is off.

* Adding support for heltec t114

And moved particularities to variant.h

* Remove old cpp comment that belongs to variant.h

It was a leftover.

* Trunk fix

---------

Co-authored-by: Tom Fifield <tom@tomfifield.net>

variants/nrf52840/heltec_mesh_node_t114/variant.h

@@ -210,6 +210,16 @@ No longer populated on PCB
 #define VBAT_AR_INTERNAL AR_INTERNAL_3_0
 #define ADC_MULTIPLIER (4.916F)
 
+// rf52840 AIN2 = Pin 4
+#define BATTERY_LPCOMP_INPUT NRF_LPCOMP_INPUT_2
+
+// We have AIN2 with a VBAT divider so AIN2 = VBAT * (100/490)
+// We have the device going deep sleep under 3.1V, which is AIN2 = 0.63V
+// So we can wake up when VBAT>=VDD is restored to 3.3V, where AIN2 = 0.67V
+// Ratio 0.67/3.3 = 0.20, so we can pick a bit higher, 2/8 VDD, which means
+// VBAT=4.04V
+#define BATTERY_LPCOMP_THRESHOLD NRF_LPCOMP_REF_SUPPLY_2_8
+
 #define HAS_RTC 0
 #ifdef __cplusplus
 }