PCI: endpoint: Add support in configfs to associate two EPCs with EPF

Now that PCI endpoint core supports to add secondary endpoint controller
(EPC) with endpoint function (EPF), Add support in configfs to associate
two EPCs with EPF. This creates "primary" and "secondary" directory inside
the directory created by users for EPF device. Users have to add a symlink
of endpoint controller (pci_ep/controllers/) to "primary" or "secondary"
directory to bind EPF to primary and secondary EPF interfaces respectively.
Existing method of linking directory representing EPF device to directory
representing EPC device to associate a single EPC device with a EPF device
will continue to work.

Link: https://lore.kernel.org/r/20210201195809.7342-8-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>