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>