Prior to this fix, `tests/extmod/vfs_lfs_mtime.py` would fail when run
after the `tests/ports/rp2/rp2_lightsleep.py` test, because
`time.time_ns()` would have a large and constant offset from `time.time()`.
Fix this by re-syncing the time-ns offset when coming out of lightsleep.
Signed-off-by: Damien George <damien@micropython.org>
120ac0f8d2