docs: Move the HTE documentation to driver-api/

The hardware timestamp engine documentation is driver API material, and
really belongs in the driver-API book; move it there.

Cc: Thierry Reding <treding@nvidia.com>
Acked-by: Dipen Patel <dipenp@nvidia.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>