net-next: dt-binding: dwmac-mediatek: add more description for RMII

MT2712 SoC can provide RMII reference clock,
so add corresponding description in dt-binding.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed