ieee802154_hal: add cap for register retention during off

[jia200x]

Contribution description

This PR adds a radio cap for indicating that the radio can keep the content of all register when off. This is required to indicate the upper layer that in some cases it might be necessary to set the PIB again.

This still leaves room for several use cases:

• Users that don't require register retention (because the CPU hibernates) and simply reboot on wake-up can ignore setting the PIB after off.
• Users that require register retention (radio sleeps but CPU is still awake) can set the PIB if the cap is not there.

It also specifies that during boot, the PIB is undefined (so it should be set once confirm_on returns 0).

I also piggybacked a misplaced note that was probably a left-over of the cleanup in #14371

Testing procedure

The output of Travis should be enough

Issues/PRs references

Discussed in #14802

[benpicco]

Good addition, only API defined so far.

[jia200x]

squashed