Implement a way to track steps when sleeping

[UnexpectedMaker]

When TinyWATCH is deep sleeping it misses the step counter interrupt so doesn't get to add the steps being done.

Let's find a way to either have them stored in IMU memory if possible, and retrieved on wake, or see if we can get the ULP going in deep sleep to bit bang the data from the IMU.

Who is going to take on this challenge?

[disq]

Looking at the BMI270 application note could a possible solution be:

• On watch sleep, set SC_26.watermark_level bits to 0 so no interrupt/output regarding the step counter is generated (because nobody will be listening) other interrupts should still be intact
• On wake up, poll it: Read SC_OUT_* registers (manually) and add to step count (maybe reset the IMU step counter before sleep or after, depending on if there are duplicate counts etc.)
• Re-enable SC_26.watermark_level so that the step counter interrupt is back

I don't think I have a BMI270 here but can't do a quick test, but could be a solution if SC_OUT can be read on-demand (and no counts are lost in between)