PCI: endpoint: Add support to link a physical function to a virtual function

While the physical function has to be linked to endpoint controller, the
virtual function has to be linked to a physical function. Add support to
link a physical function to a virtual function in pci-ep-cfs.

Link: https://lore.kernel.org/r/20210819123343.1951-4-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>