net: dsa: Add Lantiq / Intel GSWIP tag support

This handles the tag added by the PMAC on the VRX200 SoC line.

The GSWIP uses internally a GSWIP special tag which is located after the
Ethernet header. The PMAC which connects the GSWIP to the CPU converts
this special tag used by the GSWIP into the PMAC special tag which is
added in front of the Ethernet header.

This was tested with GSWIP 2.1 found in the VRX200 SoCs, other GSWIP
versions use slightly different PMAC special tags.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>