dt-bindings: net: mediatek: add optional properties for the SoC ethernet core

Introduce dma-coherent, cci-control and hifsys optional properties to
the mediatek ethernet controller 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>