Draft: Add initial BMU support
This merge request is currently finished, but is enough to allow BatMgrHAL to start and report the presence and charging state of the battery, as well as a basic percentage level.
TODO:
- Lots of functionality is still unimplemented.
- HAL device IDs are reused from other platforms since I'm not sure how those should be allocated.
- When BatMgrHAL updates the battery status, it often has a higher CPU usage than expected according to !Usage, and I'm not sure if that's because of the code in this branch or because of something else.
- The AXP803 registers ought to have proper names rather than just using the values directly.