Documentation: fpga: dfl: add descriptions for virtualization and new interfaces.

This patch adds virtualization support description for DFL based
FPGA devices (based on PCIe SRIOV), and introductions to new
interfaces added by new dfl private feature drivers.

[mdf@kernel.org: Fixed up to make it work with new reStructuredText docs]
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>