gpudev: expose GPU memory to CPU

gpudev: expose GPU memory to CPU

Enable the possibility to expose a GPU memory area and make it
accessible from the CPU.

GPU memory has to be allocated via rte_gpu_mem_alloc().

This patch allows the gpudev library to map (and unmap),
through the GPU driver, a chunk of GPU memory and to return
a memory pointer usable by the CPU to access the GPU memory area.

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