dt-bindings: net: Add missing properties used in examples

With 'unevaluatedProperties' support implemented, the following warnings
are generated in the net bindings:

Documentation/devicetree/bindings/net/qca,ar71xx.example.dt.yaml: ethernet@19000000: Unevaluated properties are not allowed ('qca,ethcfg' was unexpected)
Documentation/devicetree/bindings/net/stm32-dwmac.example.dt.yaml: ethernet@40028000: Unevaluated properties are not allowed ('reg-names', 'snps,pbl' were unexpected)
Documentation/devicetree/bindings/net/ti,cpsw-switch.example.dt.yaml: mdio@1000: Unevaluated properties are not allowed ('clocks', 'clock-names' were unexpected)
Documentation/devicetree/bindings/net/ti,k3-am654-cpsw-nuss.example.dt.yaml: mdio@f00: Unevaluated properties are not allowed ('clocks', 'clock-names' were unexpected)

Add the missing properties/nodes as necessary.

Cc: "David S. Miller" <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: "Andreas Färber" <[e-mail]>
Cc: Manivannan Sadhasivam <[e-mail]>
Cc: Maxime Coquelin <[e-mail]>
Cc: Alexandre Torgue <[e-mail]>
Cc: Nobuhiro Iwamatsu <[e-mail]>
Cc: Cristian Ciocaltea <[e-mail]>
Cc: "G. Jaya Kumaran" <[e-mail]>
Cc: Oleksij Rempel <[e-mail]>
Cc: Christophe Roullier <[e-mail]>
Cc: Grygorii Strashko <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/net/qca,ar71xx.yaml

···
         reset-names = "mac", "mdio";
         clocks = <&pll 1>, <&pll 2>;
         clock-names = "eth", "mdio";
-        qca,ethcfg = <&ethcfg>;
         phy-mode = "mii";
         phy-handle = <&phy_port4>;
     };

Documentation/devicetree/bindings/net/stm32-dwmac.yaml

···
               - st,stm32-dwmac
           - const: snps,dwmac-3.50a
 
+  reg: true
+
+  reg-names:
+    items:
+      - const: stmmaceth
+
   clocks:
     minItems: 3
     items:

Documentation/devicetree/bindings/net/ti,davinci-mdio.yaml

···
     maximum: 2500000
     description: MDIO Bus frequency
 
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: fck
+
   ti,hwmods:
     description: TI hwmod name
     deprecated: true