Documentation: PCI: Guide to use PCI Endpoint Core Layer

Add Documentation to help users use endpoint library to enable endpoint
mode in the PCI controller and add new PCI endpoint functions.

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>