net: ethernet: mtk_eth_soc: add flow offloading support

This adds support for offloading IPv4 routed flows, including SNAT/DNAT,
one VLAN, PPPoE and DSA.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>