net: micrel: Change to receive timestamp in the frame for lan8841

Currently for each timestamp frame, the SW needs to go and read the
received timestamp over the MDIO bus. But the HW has the capability
to store the received nanoseconds part and the least significant two
bits of the seconds in the reserved field of the PTP header. In this
way we could save few MDIO transactions (actually a little more
transactions because the access to the PTP registers are indirect)
for each received frame.

Instead of reading the rest of seconds part of the timestamp of the
frame using MDIO transactions schedule PTP worker thread to read the
seconds part every 500ms and then for each of the received frames use
this information. Because if for example running with 512 frames per
second, there is no point to read 512 times the second part.

Doing all these changes will give a great CPU usage performance.
Running ptp4l with logSyncInterval of -9 will give a ~60% CPU
improvement.
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>