net: gptp: adjust only frequency for continuous synchronization

Current clock synchronization was always stepping clock. This was
causing large offset, and discontiguous ptp hardware clock time.
For TSN hardware, discontiguous ptp hardware clock time was not
able to be used for other TSN protocols.

This patch is to convert to frequency adjustment with a basic
PI control algorithm.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>