init Nvidia RDMA/ PCIe P2P Support

[sjkelly]

This adds support for Nvidia RDMA to allow for PCIe P2P transfers between the a LiteX device and a Nvidia GPU.

The basic API is documented here: https://docs.nvidia.com/cuda/gpudirect-rdma/index.html

This has been tested with the open source Nvidia drivers: https://github.com/NVIDIA/open-gpu-kernel-modules

The feature is selected in user/kernel by setting:

make NV_DMA=true

Co-authored-by: Tim Besard [email protected] Co-authored-by: Elliot Saba [email protected]