arm64: dts: mt2712: update ethernet device node

Since there are some changes in ethernet driver:
update ethernet device node in dts to accommodate to it.

1. stmmac_probe_config_dt() in stmmac_platform.c will initialize specified
   parameters according to compatible string "snps,dwmac-4.20a", then,
   dwmac-mediatek.c can skip the initialization if add compatible string
   "snps,dwmac-4.20a" in eth device node.
2. commit 882007ed ("net-next: dt-binding: dwmac-mediatek: add more
   description for RMII") added rmii internal support, we should add
   corresponding clocks/clocks-names in eth device node.
3. add "snps,reset-delays-us = <0 10000 10000>;" to ensure reset delay
   can meet PHY requirement.

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