dt-bindings: net: mediatek,net: add missing mediatek,mt7621-eth

Document the Ethernet controller found in the MediaTek MT7621 MIPS SoC
family which is supported by the mtk_eth_soc driver.

Fixes: 889bcbdeee ("net: ethernet: mediatek: support MT7621 SoC ethernet hardware")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/ec4371c4b5a331c5217b5f13a0c9e6c444838e14.1690246066.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -19,6 +19,7 @@ properties:
     enum:
       - mediatek,mt2701-eth
       - mediatek,mt7623-eth
+      - mediatek,mt7621-eth
       - mediatek,mt7622-eth
       - mediatek,mt7629-eth
       - mediatek,mt7981-eth
@@ -32,7 +33,7 @@ properties:
   clock-names: true
 
   interrupts:
-    minItems: 3
+    minItems: 1
     maxItems: 4
 
   power-domains:
@@ -131,6 +132,30 @@ allOf:
 
         mediatek,wed-pcie: false
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - mediatek,mt7621-eth
+    then:
+      properties:
+        interrupts:
+          maxItems: 1
+
+        clocks:
+          minItems: 2
+          maxItems: 2
+
+        clock-names:
+          items:
+            - const: ethif
+            - const: fe
+
+        mediatek,wed: false
+
+        mediatek,wed-pcie: false
+
   - if:
       properties:
         compatible: