PCI: endpoint: Add EP function driver to provide NTB functionality

Add a new endpoint function driver to provide NTB functionality using
multiple PCIe endpoint instances.

[arnd@arndb.de: Select configfs dependency]
[yebin10@huawei.com: Fix unused but set variables]
[geert+renesas@glider.be: Explain NTB in PCI_EPF_NTB help text]

Link: https://lore.kernel.org/r/20210201195809.7342-14-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>