net: airoha: Introduce flowtable offload support

Introduce netfilter flowtable integration in order to allow airoha_eth
driver to offload 5-tuple flower rules learned by the PPE module if the
user accelerates them using a nft configuration similar to the one reported
below:

table inet filter {
	flowtable ft {
		hook ingress priority filter
		devices = { lan1, lan2, lan3, lan4, eth1 }
		flags offload;
	}
	chain forward {
		type filter hook forward priority filter; policy accept;
		meta l4proto { tcp, udp } flow add @ft
	}
}

Tested-by: Sayantan Nandy <sayantan.nandy@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>