net: pkt: time: introduce ns timestamp helper

A little refactoring that simplifies dealing with nanosecond timestamp
values in packets and further decouples calling code from PTP:

Benefits:
- simplifies calling code by removing redundant conversions.
- prepares for removing PTP dependencies from net_pkt.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>