include: drivers: ptp_clock: clarify ptp_clock_rate_adjust function

It's not very clear about the function of ptp_clock_rate_adjust in doc.
Previously, all device drivers used it to adjust rate ratio relatively
based on current frequency.

When PI servo was introduced in PTP and gPTP stacks, NXP ENET PTP driver
started to use it to adjust rate ratio based on its nominal frequency.

Rate ratio adjustment based on nominal frequency with PI servo could
get stable frequency control. So, let's clarify ptp_clock_rate_adjust
adjusting rate ratio based on nominal frequency, and convert other
device drivers.

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