sfc: Add PTP counters to ethtool stats

These were implemented by Andrew Jackson and Laurence Evans but not
previously included in-tree.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

diff --cc drivers/net/ethernet/sfc/ethtool.c
Simple merge

diff --cc drivers/net/ethernet/sfc/nic.h
Simple merge

diff --cc drivers/net/ethernet/sfc/ptp.c
Simple merge