net: ethernet: mtk_eth_soc: add support for Wireless Ethernet Dispatch (WED)

The Wireless Ethernet Dispatch subsystem on the MT7622 SoC can be
configured to intercept and handle access to the DMA queues and
PCIe interrupts for a MT7615/MT7915 wireless card.
It can manage the internal WDMA (Wireless DMA) controller, which allows
ethernet packets to be passed from the packet switch engine (PSE) to the
wireless card, bypassing the CPU entirely.
This can be used to implement hardware flow offloading from ethernet to
WLAN.

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