net: gptp: fix follow_up message timestamp

According to IEEE802.1AS 11.4.4.2.1, we need fill
preciseOriginTimestamp as syncEventEgressTimestamp.

In this follow_up message, prec_orig_ts need to filled
from net_pkt_timestamp(sync) for best accuracy. state machine
is software trigger with insufficient precision.

After this change, a grand master endpoint sync accuracy
increase from 3.5ms to 580ns with mimxrt1050_evk board.

Signed-off-by: Chen Caidy <chen@caidy.cc>