Documentation: PCI: Add specification for the PCI NTB function device

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

[bhelgaas: fix a few typos]
Link: https://lore.kernel.org/r/20210201195809.7342-2-kishon@ti.comSigned-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/PCI/endpoint/index.rst

@@ -11,5 +11,6 @@ PCI Endpoint Framework
    pci-endpoint-cfs
    pci-test-function
    pci-test-howto
+   pci-ntb-function
 
    function/binding/pci-test