Merge branch 'ocelot-ptp'

Yangbo Lu says:

====================
Support Ocelot PTP Sync one-step timestamping

This patch-set is to support Ocelot PTP Sync one-step timestamping.
Actually before that, this patch-set cleans up and optimizes the
DSA slave tx timestamp request handling process.

Changes for v2:
	- Split tx timestamp optimization patch.
	- Updated doc patch.
	- Freed skb->cb usage in dsa core driver, and moved to device
	  drivers.
	- Other minor fixes.
Changes for v3:
	- Switched sequence of patch #3 and #4 with rebasing to fix build.
	- Replaced hard coded 48 of memset(skb->cb, 0, 48) with sizeof().
====================

Signed-off-by: David S. Miller <davem@davemloft.net>