Implement Battery Monitoring for promicro

[todd-herbert]

A few tweaks to get battery monitoring working for the promicro variant:

• modify equation in Promicroboard::getBattMilliVolts
• correct AREF_VOLTAGE, lower ADC_MULTIPLIER accordingly
• raise ADC_RESOLUTION

[mattzzw]

Regading the default adc multiplier value:
Which promicro variant are you referring to?
The FakeTec boards for example use resistors of equal values for the adc voltage devider.

[todd-herbert]

Regading the default adc multiplier value: Which promicro variant are you referring to? The FakeTec boards for example use resistors of equal values for the adc voltage devider.

Oh yes, sorry, well spotted!
I'd previously been using faketec boards with Meshtastic, which for some reason expects a 1MOhm-1.5MOhm divider.

It totally slipped my mind that this was abnormal. I'll update to reflect the official faketec setup with the two equal divider resistors.