net: zynq: gem: Use hardware assisted system time stamping

Since the hardware frequency adjustment is complex,using software
method to do that.

In this mode, PTP packets are still time stamped by the NIC, but the clock
which is controlled by PTPd is the system time. For this to work, the time
stamps generated inside the NIC are transformed into system time before
feeding them into the normal PTPd machinery: each time a NIC time stamp is
extracted from the NIC, the NIC system time offset is measured and added to
the NIC time stamp to obtain the corresponding system time stamp.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>