Add binding documentation for pci-ntb endpoint function that helps in
adding and configuring pci-ntb endpoint function.

Link: https://lore.kernel.org/r/20210201195809.7342-17-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>