net: stmmac: Add support for VLAN Insertion Offload

Adds the logic to insert a given VLAN ID in a packet. This is offloaded
to HW and its descriptor based. For now, only XGMAC implements the
necessary callbacks.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>