net: dsa: hellcreek: Add support for hardware timestamping

net: dsa: hellcreek: Add support for hardware timestamping

The switch has the ability to take hardware generated time stamps per port for
PTPv2 event messages in Rx and Tx direction. That is useful for achieving needed
time synchronization precision for TSN devices/switches. So add support for it.

There are two directions:

 * RX

   The switch has a single register per port to capture a timestamp. That
   mechanism is not used due to correlation problems. If the software processing
   is too slow and a PTPv2 event message is received before the previous one has
   been processed, false timestamps will be captured. Therefore, the switch can
   do "inline" timestamping which means it can insert the nanoseconds part of
   the timestamp directly into the PTPv2 event message. The reserved field (4
   bytes) is leveraged for that. This might not be in accordance with (older)
   PTP standards, but is the only way to get reliable results.

 * TX

   In Tx direction there is no correlation problem, because the software and the
   driver has to ensure that only one event message is "on the fly". However,
   the switch provides also a mechanism to check whether a timestamp is
   lost. That can only happen when a timestamp is read and at this point another
   message is timestamped. So, that lost bit is checked just in case to indicate
   to the user that the driver or the software is somewhat buggy.

Signed-off-by: Kamil Alkhouri <kamil.alkhouri@hs-offenburg.de>
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>