NTB: EPF: support NTB transfer between PCI RC and EP connection

Add NTB function driver and virtual PCI BUS and Virtual NTB driver
to implement communication between PCIe RC and PCIe EP devices

┌────────────┐         ┌─────────────────────────────────────┐
│            │         │                                     │
├────────────┤         │                      ┌──────────────┤
│ NTB        │         │                      │ NTB          │
│ NetDev     │         │                      │ NetDev       │
├────────────┤         │                      ├──────────────┤
│ NTB        │         │                      │ NTB          │
│ Transfer   │         │                      │ Transfer     │
├────────────┤         │                      ├──────────────┤
│            │         │                      │              │
│  PCI NTB   │         │                      │              │
│    EPF     │         │                      │              │
│   Driver   │         │                      │ PCI Virtual  │
│            │         ├───────────────┐      │ NTB Driver   │
│            │         │ PCI EP NTB    │◄────►│              │
│            │         │  FN Driver    │      │              │
├────────────┤         ├───────────────┤      ├──────────────┤
│            │         │               │      │              │
│  PCI BUS   │ ◄─────► │  PCI EP BUS   │      │  Virtual PCI │
│            │  PCI    │               │      │     BUS      │
└────────────┘         └───────────────┴──────┴──────────────┘
    PCI RC                        PCI EP

This driver include 3 part:
 1 PCI EP NTB function driver
 2 Virtual PCI bus
 3 PCI virutal NTB driver, which is loaded only by above virtual pci bus

Signed-off-by: Frank Li <Frank.Li@nxp.com>

drivers/pci/endpoint/functions/Kconfig

@@ -25,3 +25,14 @@ config PCI_EPF_NTB
 	  device tree.
 
 	  If in doubt, say "N" to disable Endpoint NTB driver.
+
+config PCI_EPF_VNTB
+        tristate "PCI Endpoint NTB driver"
+        depends on PCI_ENDPOINT
+        select CONFIGFS_FS
+        help
+          Select this configuration option to enable the Non-Transparent
+          Bridge (NTB) driver for PCI Endpoint. NTB driver implements NTB
+          between PCI host and PCIe Endpoint.
+
+          If in doubt, say "N" to disable Endpoint NTB driver.

drivers/pci/endpoint/functions/Makefile

@@ -5,3 +5,4 @@
 
 obj-$(CONFIG_PCI_EPF_TEST)		+= pci-epf-test.o
 obj-$(CONFIG_PCI_EPF_NTB)		+= pci-epf-ntb.o
+obj-$(CONFIG_PCI_EPF_VNTB) 		+= pci-epf-vntb.o