ipq40xx: mdio: remove support for phy-reset-gpio

ipq40xx: mdio: remove support for phy-reset-gpio

Commit 6f6c00cfc95f ("ipq40xx: Add support for Unielec U4019") has
introduced support for `phy-reset-gpio` DT property, which isn't needed
as the MDIO already supports `reset-gpios`[1] which could be used instead.

1. https://elixir.bootlin.com/linux/v4.19.81/source/Documentation/devicetree/bindings/net/mdio.txt

Ref: PR#2511
Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>
[commit title and description facelift]
Signed-off-by: Petr Štetiar <ynezz@true.cz>