net: add support for Cavium PTP coprocessor

This patch adds support for the Precision Time Protocol
Clocks and Timestamping hardware found on Cavium ThunderX
processors.

Signed-off-by: Radoslaw Biernacki <rad@semihalf.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@cavium.com>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>