i40e: adds FCoE to build and updates its documentation

Adds newly added FCoE files to the build but only if FCoE module is configured.

Also, updates i40e document for added FCoE support.

Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Tested-by: Jack Morgan<jack.morgan@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>