Implement the ethtool private statistics interface to expose additional
port-level and MAC-level counters that are not covered by the standard
IEEE 802.3 statistics. The pMAC counters are only reported when the port
supports Frame Preemption (802.1Qbu/802.3br).

Note that although rtnl_link_stats64 provides some standard statistics
such as rx octets, rx frame errors, rx dropped packets, and tx packets,
these are overall port statistics. The NETC switch supports preemption
on each port, and each port has two MACs (eMAC and pMAC). The driver
private statistics are used to obtain statistics for each MAC, allowing
users to perform analysis and debugging.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20260518082506.1318236-16-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Each user port of the NETC switch supports 802.3 basic and mandatory
managed objects statistic counters and IETF Management Information
Database (MIB) package (RFC2665) and Remote Network Monitoring (RMON)
counters. And all of these counters are 64-bit registers. In addition,
some user ports support preemption, so these ports have two MACs, MAC
0 is the express MAC (eMAC), MAC 1 is the preemptible MAC (pMAC). So
for ports that support preemption, the statistics are the sum of the
pMAC and eMAC statistics.

Note that the current switch driver does not support preemption, all
frames are sent and received via the eMAC by default. The statistics
read from the pMAC should be zero.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20260518082506.1318236-15-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>