Power management options for BMI270 driver

[Kampi]

ToDo

• Proove that the sensor is working when power saving options are enabled
• Add driver code for power management
• Sensor feature enabling / disabling during runtime

[jakkra]

Low prio, as it's already consuming quite little and as long as rest of the system is much more than BMI270, getting other parts of ZSWatch lower power is higher prio.

[Kampi]

Low prio, as it's already consuming quite little and as long as rest of the system is >> BMI270, getting other parts of ZSWatch lower power is higher prio.

Yes. Just to get it out of my head and as a collection list for features :)