sys/ztimer: add ZTIMER_NSEC from PTP backend

[fabian18]

Contribution description

This PR adds a ZTIMER_NSEC clock, based on periph_ptp.

Testing procedure

Try tests/ztimer_xsec

Issues/PRs references

#16309

[kfessel]

looks good in huge parts, :+1:

But most boards do not provide periph_ptp -> the test ztimer_xsec will fail for most boards (inkl. native).

The test needs to adapt either the board or to the used modules. (or not test NSEC by default)

[fabian18]

Now that ztimer64 is there, should I add MODULE_ZTIMER64_NSEC in this PR or should it be another PR?

[stale[bot]]

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