Add support for 3-cell battery on Librem 14
- Implement and check for 3-cell battery compatibility, 3-cell is 3P, 4-cell is 2S2P
- Add emergency power down if battery voltage drops below minimum
- Fix system voltage setting based on 3/4-cell battery
Note: this MR is stacked on top of !13 (merged); MR !13 (merged) should be merged first, then this.
Signed-off-by: Matt DeVillier matt.devillier@puri.sm