dt-bindings: net: dsa: mediatek,mt7530: Add missing 'reg' property

The 'reg' property is missing from the mediatek,mt7530 schema which
results in the following warning once 'unevaluatedProperties' is fixed:

Documentation/devicetree/bindings/net/dsa/mediatek,mt7530.example.dtb: switch@0: Unevaluated properties are not allowed ('reg' was unexpected)

Fixes: e0dda3119741 ("dt-bindings: net: dsa: convert binding for mediatek switches")
Signed-off-by: Rob Herring <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/dsa/mediatek,mt7530.yaml

···
       - mediatek,mt7531
       - mediatek,mt7621
 
+  reg:
+    maxItems: 1
+
   core-supply:
     description:
       Phandle to the regulator node necessary for the core power.