net: dsa: add PTP solution in core driver

Added PTP solution in core driver. Now only gPTP was supported.

- ethernet_api like get_capabilities and get_ptp_clock
  were supported in port driver.

- For TX timestamp, added new dsa_api port_txtstamp for device
  to handle. Device driver might put pkt in queue, and reported
  timestamp after completing transmitting on hardware.

- For RX timestamp, the timestamp could be given to pkt in tag
  driver or device driver.

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