dt-bindings: Add missing 'properties' keyword enclosing 'snps,tso'

DT property definitions must be under a 'properties' keyword. This was
missing for 'snps,tso' in an if/then clause. A meta-schema fix will
catch future errors like this.

Fixes: 7db3545a ("dt-bindings: net: stmmac: Convert the binding to a schemas")
Cc: "David S. Miller" <davem@davemloft.net>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/net/snps,dwmac.yaml

@@ -347,6 +347,7 @@ allOf:
               - st,spear600-gmac
 
     then:
+      properties:
         snps,tso:
           $ref: /schemas/types.yaml#definitions/flag
           description: