dt-bindings: dwmac: Add bindings for new Loongson SoC and bridge chip

Add the dwmac bindings for the Loongson-2K SoC and the LS7A
bridge chip.

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>