net: ptp: extract PTP interfaces outside gPTP subsys under NET_L2_PTP

As per #38352, we would like to start building out PTP (IEEE 1588)
support for superset of gPTP functionality in Zephyr. This is the first
step to abstract away some key interfaces from NET_GPTP umbrella to
NET_L2_PTP.

Signed-off-by: Alex Sergeev <asergeev@carbonrobotics.com>