dt-bindings: net: mediatek,net: fixup MAC binding

1. Use unevaluatedProperties
It's needed to allow ethernet-controller.yaml properties work correctly.

2. Drop unneeded phy-handle/phy-mode

3. Don't require phy-handle
Some SoCs may use fixed link.

For in-kernel MT7621 DTS files this fixes following errors:
arch/mips/boot/dts/ralink/mt7621-tplink-hc220-g5-v1.dtb: ethernet@1e100000: mac@0: 'fixed-link' does not match any of the regexes: 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/net/mediatek,net.yaml
arch/mips/boot/dts/ralink/mt7621-tplink-hc220-g5-v1.dtb: ethernet@1e100000: mac@0: 'phy-handle' is a required property
        From schema: Documentation/devicetree/bindings/net/mediatek,net.yaml
arch/mips/boot/dts/ralink/mt7621-tplink-hc220-g5-v1.dtb: ethernet@1e100000: mac@1: 'fixed-link' does not match any of the regexes: 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/net/mediatek,net.yaml
arch/mips/boot/dts/ralink/mt7621-tplink-hc220-g5-v1.dtb: ethernet@1e100000: mac@1: 'phy-handle' is a required property
        From schema: Documentation/devicetree/bindings/net/mediatek,net.yaml

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>