ethdev: UDP tunnels

Add two functions to support UDP tunneling port configuration.

There are "some" destination UDP port numbers that have unique meaning.
In terms of VxLAN, "IANA has assigned the value 4789 for the VXLAN UDP port,
and this value SHOULD be used by default as the destination UDP port.
Some early implementations of VXLAN have used other values for the destination
port. To enable interoperability with these implementations, the destination
port SHOULD be configurable."

Signed-off-by: Jijiang Liu <jijiang.liu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>
Tested-by: Yong Liu <yong.liu@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>