PCI: endpoint: functions: Add an EP function to test PCI

Adds a new endpoint function driver (to program the virtual test device)
making use of the EP-core library.

[bhelgaas: fold in pci_epf_test_probe() -ENOMEM test from Wei Yongjun
<weiyongjun1@huawei.com>]
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>