PCI: endpoint: Add support to add virtual function in endpoint core

Add support to add virtual function in endpoint core. The virtual
function can only be associated with a physical function instead of a
endpoint controller. Provide APIs to associate a virtual function with
a physical function here.

[weiyongjun1@huawei.com: PCI: endpoint: Fix missing unlock on error in
pci_epf_add_vepf() - Reported-by: Hulk Robot <hulkci@huawei.com&gt;]>

Link: https://lore.kernel.org/r/20210819123343.1951-3-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>