net/mlx5e: Add device PCIe congestion ethtool stats

Implement the PCIe Congestion Event notifier which triggers a work item
to query the PCIe Congestion Event object. The result of the congestion
state is reflected in the new ethtool stats:

* pci_bw_inbound_high: the device has crossed the high threshold for
  inbound PCIe traffic.
* pci_bw_inbound_low: the device has crossed the low threshold for
  inbound PCIe traffic
* pci_bw_outbound_high: the device has crossed the high threshold for
  outbound PCIe traffic.
* pci_bw_outbound_low: the device has crossed the low threshold for
  outbound PCIe traffic

The high and low thresholds are currently configured at 90% and 75%.
These are hysteresis thresholds which help to check if the
PCI bus on the device side is in a congested state.

If low + 1 = high then the device is in a congested state. If low == high
then the device is not in a congested state.

The counters are also documented.

A follow-up patch will make the thresholds configurable.

Signed-off-by: Dragos Tatulea <[email protected]>
Signed-off-by: Tariq Toukan <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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