Form: clearValidate supports string type param (#12990)

packages/form/src/form.vue

@@ -85,7 +85,10 @@
       },
       clearValidate(props = []) {
         const fields = props.length
-          ? this.fields.filter(field => props.indexOf(field.prop) > -1)
+          ? (typeof props === 'string' 
+              ? this.fields.filter(field => props === field.prop)
+              : this.fields.filter(field => props.indexOf(field.prop) > -1)
+            )
           : this.fields;
         fields.forEach(field => {
           field.clearValidate();