PCI: endpoint: Introduce configfs entry for configuring EP functions

Introduce a new configfs entry to configure the EP function (like
configuring the standard configuration header entries) and to bind the EP
function with EP controller.

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