dpaa2-eth: add APIs of 1588 single step timestamping

This patch is to add APIs of 1588 single step timestamping.

- dpni_set_single_step_cfg
- dpni_get_single_step_cfg

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>