NTB: Add support for EPF PCI Non-Transparent Bridge

NTB: Add support for EPF PCI Non-Transparent Bridge

Add support for EPF PCI Non-Transparent Bridge (NTB) devices.  This driver
is platform independent and may be used by any platform that has multiple
PCI endpoint instances configured using the pci-epf-ntb driver.  The driver
connnects to the standard NTB subsystem interface. The EPF NTB device has a
configurable number of memory windows (max 4), a configurable number of
doorbells (max 32), and a configurable number of scratch-pad registers.

Link: https://lore.kernel.org/r/20210201195809.7342-16-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: Dave Jiang <dave.jiang@intel.com>