i40evf: add driver to kernel build system

Modify the existing Kconfig, Makefile, and MAINTAINERS to add the driver
to the kernel. Add a Makefile and a documentation

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

diff --cc Documentation/networking/i40evf.txt
Simple merge

diff --cc MAINTAINERS
Simple merge

diff --cc drivers/net/ethernet/intel/Kconfig
Simple merge

diff --cc drivers/net/ethernet/intel/Makefile
Simple merge

diff --cc drivers/net/ethernet/intel/i40evf/Makefile
Simple merge