PCI: endpoint: Add EP core layer to enable EP controller and EP functions

Introduce a new EP core layer in order to support endpoint functions in
linux kernel. This comprises the EPC library (Endpoint Controller Library)
and EPF library (Endpoint Function Library). EPC library implements
functions specific to an endpoint controller and EPF library implements
functions specific to an endpoint function.

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>