net: ethernet: mtk_eth_soc: add support for initializing the PPE

The PPE (packet processing engine) is used to offload NAT/routed or even
bridged flows. This patch brings up the PPE and uses it to get a packet
hash. It also contains some functionality that will be used to bring up
flow offloading.

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>