gpu/cuda: map GPU memory with GDRCopy

gpu/cuda: map GPU memory with GDRCopy

To enable the gpudev rte_gpu_mem_cpu_map feature to expose
GPU memory to the CPU, the GPU CUDA driver library needs
the GDRCopy library and driver.

If DPDK is built without GDRCopy, the GPU CUDA driver returns
error if the is invoked rte_gpu_mem_cpu_map.

All the others GPU CUDA driver functionalities are not affected by
the absence of GDRCopy, thus this is an optional functionality
that can be enabled in the GPU CUDA driver.

CUDA driver documentation has been updated accordingly.

Signed-off-by: Elena Agostini <eagostini@nvidia.com>