net: dt-bindings: dwmac: Convert mediatek-dwmac to DT schema

Convert mediatek-dwmac to DT schema, and delete old mediatek-dwmac.txt.
And there are some changes in .yaml than .txt, others almost keep the same:
  1. compatible "const: snps,dwmac-4.20".
  2. delete "snps,reset-active-low;" in example, since driver remove this
     property long ago.
  3. add "snps,reset-delay-us = <0 10000 10000>" in example.
  4. the example is for rgmii interface, keep related properties only.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>