PCI: trace: Add RAS tracepoint to monitor link speed changes

PCI: trace: Add RAS tracepoint to monitor link speed changes

PCIe link speed degradation directly impacts system performance and often
indicates hardware issues such as faulty devices, physical layer problems,
or configuration errors.

To this end, add a RAS tracepoint to monitor link speed changes, enabling
proactive health checks and diagnostic analysis.

The following output is generated when a device is hotplugged:

  $ echo 1 > /sys/kernel/debug/tracing/events/pci/pcie_link_event/enable
  $ cat /sys/kernel/debug/tracing/trace_pipe
     irq/51-pciehp-88      [001] .....   381.545386: pcie_link_event: 0000:00:02.0 type:4, reason:4, cur_bus_speed:20, max_bus_speed:23, width:1, flit_mode:0, status:DLLLA

Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Suggested-by: Matthew W Carlis <mattc@purestorage.com>
Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/20251210132907.58799-3-xueshuai@linux.alibaba.com