Define BATTERY_SENSE_RESOLUTION_BITS based on [amoroz's snippet on the Meshtastic forum](https://meshtastic.discourse.group/t/new-1w-diy-variant-xiao-nrf52840-ebyte-e22-900m30s/7904/10).
Fixes following build error:
```
src/Power.cpp: In member function 'virtual uint16_t AnalogBatteryLevel::getBattVoltage()':
src/Power.cpp:224:79: error: 'BATTERY_SENSE_RESOLUTION_BITS' was not declared in this scope
scaled = operativeAdcMultiplier * ((1000 * AREF_VOLTAGE) / pow(2, BATTERY_SENSE_RESOLUTION_BITS)) * raw;
```
Signed-off-by: Andrew Yong <me@ndoo.sg>
f8c3f43ea6