cxl: Compute the entire CXL path latency and bandwidth data

CXL Memory Device SW Guide [1] rev1.0 2.11.2 provides instruction on how to
calculate latency and bandwidth for CXL memory device. Calculate minimum
bandwidth and total latency for the path from the CXL device to the root
port. The QTG id is retrieved by providing the performance data as input
and calling the root port callback ->get_qos_class(). The retrieved id is
stored with the cxl_port of the CXL device.

For example for a device that is directly attached to a host bus:
Total Latency = Device Latency (from CDAT) + Dev to Host Bus (HB) Link
		Latency + Generic Port Latency
Min Bandwidth = Min bandwidth for link bandwidth between HB
		and CXL device, device CDAT bandwidth, and Generic Port
		Bandwidth

For a device that has a switch in between host bus and CXL device:
Total Latency = Device (CDAT) Latency + Dev to Switch Link Latency +
		Switch (CDAT) Latency + Switch to HB Link Latency +
		Generic Port Latency
Min Bandwidth = Min bandwidth for link bandwidth between CXL device
		to CXL switch, CXL device CDAT bandwidth, CXL switch CDAT
		bandwidth, CXL switch to HB bandwidth, and Generic Port
		Bandwidth.

[1]: https://cdrdv2-public.intel.com/643805/643805_CXL%20Memory%20Device%20SW%20Guide_Rev1p0.pdf

Reviewed-by: Jonathan Cameron <[email protected]>
Signed-off-by: Dave Jiang <[email protected]>
Link: https://lore.kernel.org/r/170319624458.2212653.13252496567443656371.stgit@djiang5-mobl3
Signed-off-by: Dan Williams <[email protected]>

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