net: ethernet: mtk_eth_soc: add support for coherent DMA

It improves performance by eliminating the need for a cache flush on rx and tx
In preparation for supporting WED (Wireless Ethernet Dispatch), also add a
function for disabling coherent DMA at runtime.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: David S. Miller <davem@davemloft.net>