AXP192 Multiple Fixes and Comments.

[ryanzmi]

This is my first time to do PR.. But I do like to improve the lovely M5stickC. Pls let me know if I do it wrong.

• Set Power Off Volt to 3.0v when init.
• Add GetInputPowerStatus()
• Add GetBatteryChargingStatus()
• ClearCoulombcounter() fix to only set clear flag
• Fix GetCoulombData() when coin < coout, the uint32 failed to show the correct negative value (arithmetic error when trying subtract unsigned value.)
• Add Read6BytesStorage() and Write6BytesStorage() from Axp. Useful for Battery Capacity saving.
• Remove tabs, replace with space and add some useful comments.