Documentation: PCI: Guide to use PCI endpoint configfs

Add Documentation to help users use PCI endpoint to configure PCI endpoint
function and to bind the endpoint function with endpoint controller.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-By: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>