zephyr: time: Add "now" for Instant

[aolowin]

Uses the full tick resolution rather than a millisecond conversion.

[d3zd3z]

Thanks for your contribution. I did already add this in the "work q" PR, but I will add an acknowledgement to your contribution there. I think you are right in only really providing this function when time is 64-bit, as Instant just doesn't make a lot of sense with an overflowing timer.