net: if: Add TX timestamp callback support

Allow creation of TX timestamp thread which will collect TX timestamp
information from device drivers. If the callback is registered, then
it will pass that timestamp information to the relevant party for
further processing. This support will be used by gPTP code in
subsequent commits.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Julien Chevrier <julien.chevrier@intel.com>