amd-xgbe: Add support for VXLAN offload capabilities

The hardware has the capability to perform checksum offload support
(both Tx and Rx) and TSO support for VXLAN packets. Add the support
required to enable this.

The hardware can only support a single VXLAN port for offload. If more
than one VXLAN port is added then the offload capabilities have to be
disabled and can no longer be advertised.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>