dt-bindings: arm: mediatek: document the pcie mirror node on MT7622

This patch adds the pcie mirror document bindings for MT7622 SoC.
The feature is used for intercepting PCIe MMIO access for the WED core
Add related info in mediatek-net bindings.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: David S. Miller <davem@davemloft.net>