net: ethernet: mtk_eth_soc: add the capability to run multiple ppe

mt7986 chipset support multiple packet engines for wlan <-> eth
packet forwarding.

Tested-by: Daniel Golle <daniel@makrotopia.org>
Co-developed-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Co-developed-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>