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.com


Signed-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>