Documentation: PCI: Add PCI endpoint NTB function user guide

Add documentation to help users use pci-epf-ntb function driver and
existing host side NTB infrastructure for NTB functionality.

[bhelgaas: fix a few typos]
Link: https://lore.kernel.org/r/20210201195809.7342-18-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>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>