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>