Documentation: PCI: Add specification for the *PCI test* function device

Add specification for the *PCI test* virtual function device. The endpoint
function driver and the host PCI driver should be created based on this
specification.

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