PCI: endpoint: Add virtual function number in pci_epc ops

Add virtual function number in pci_epc ops. EPC controller driver
can perform virtual function specific initialization based on the
virtual function number.

Link: https://lore.kernel.org/r/20210819123343.1951-5-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>