Enabling peer to peer device transactions for PCIe devices

[Serguei Sagalovitch]

> A white list may end up being rather complicated if it has to cover
> different CPU generations and system architectures. I feel this is a
> decision user space could easily make.
>
> Logan
I agreed that it is better to leave up to user space to check what is 
working
and what is not. I found that write is practically always working but 
read very
often not. Also sometimes system BIOS update could fix the issue.