[coreboot] Why is subsystemid required to find a uPD7020200

[Charlotte Plusplus]

Hello

The USB3 on a W520 is provided by a uPD7020200 on PCIe port 7, so I just
had in my device tree:
    device pci 1c.6 on end

Yet in the boot I was getting:
     PCI: Static device PCI: 00:1c.6 not found, disabling it.

After googlging about this, I found the x230 uses subsystemid
     device pci 1c.6 on subsystemid 0x17aa 0x21db end

After trying that, I could get the USB3 to be detected and working
correctly on the W520.

Nowhere else I need subsystemid, so I don't understand why it is needed
here.
I don't want to copy something I don't understand, especially if it may be
needed in other places.

Could someone explain when subsystemid is required?

Thanks
Charlotte
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