Add a sysfs file that reports the number of bytes transmitted and
received in the last second. This can be used to approximate the PCIe
bandwidth usage over the last second.

v2: Clarify use of mps as estimation of bandwidth
v3: Don't make the file on APUs
v4: Early exit for APUs in the read function, change output to
    display "packets-received packets-sent mps"
v5: fix missing header for si (Alex)
Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>