net: ethernet: mtk_eth_soc: implement dynamic interrupt moderation

Reduces the number of interrupts under load

Signed-off-by: Felix Fietkau <nbd@nbd.name>
[Ilya: add documentation for new struct fields]
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>