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 <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/mediatek,net.yaml

···
 patternProperties:
   "^mac@[0-1]$":
     type: object
-    additionalProperties: false
+    unevaluatedProperties: false
     allOf:
       - $ref: ethernet-controller.yaml#
     description:
···
       reg:
         maxItems: 1
 
-      phy-handle: true
-
-      phy-mode: true
-
     required:
       - reg
       - compatible
-      - phy-handle
 
 required:
   - compatible