PCI: endpoint: Support NTB transfer between RC and EP

Add NTB function driver and virtual PCI Bus and Virtual NTB driver
to implement communication between PCIe Root Port 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      │
└────────────┘         └───────────────┴──────┴──────────────┘
PCIe Root Port                        PCI EP

This driver includes 3 parts:
 1 PCI EP NTB function driver
 2 Virtual PCI bus
 3 PCI virtual NTB driver, which is loaded only by above virtual PCI bus

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>