doc: add known issue for i40e PHY link up
doc: add known issue for i40e PHY link up

Some kernel drivers are not able to handle the link status correctly
after DPDK application sets the PHY to link down.

Signed-off-by: Jeff Guo <62aa901f8680899d1e8b3d9b45cf4843c2976e1c@intel.com>
Acked-by: Marko Kovacevic <32a723043e16bf2a3185e32c2e613ae1ee55288f@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk