rp2040: rtc deep sleep

[ysoldak]

Builds on top of #3403 (hence includes a commit from there)

This PR implements deep sleep (not dormant state!) on RP2040.
Example added that shows how to use the feature with some power consumption stats for Nano RP2040 Connect board.

Example output

1970-01-01T00:00:05Z Base power, 5 sec, 0.11W
1970-01-01T00:00:10Z High power, 5 sec, 0.13W
1970-01-01T00:00:15Z  Low power, 5 sec, 0.03W
1970-01-01T00:00:15Z Base power, 5 sec, 0.11W
1970-01-01T00:00:20Z High power, 5 sec, 0.13W
1970-01-01T00:00:25Z  Low power, 5 sec, 0.03W
1970-01-01T00:00:25Z Base power, 5 sec, 0.11W
1970-01-01T00:00:30Z High power, 5 sec, 0.13W
1970-01-01T00:00:35Z  Low power, 5 sec, 0.03W

Observe how internal time (accessed by time.Now()) stops while system is in deep sleep.

Draft, because I still observe Serial console problems after wake up.
The application does work, judging by on-board led going on and off and measured power consumption, but output in Serial stops. No disconnects though, just nothing prints.