schema: Add graphics element passwdValidTo attribute to schema

We support this in code, but forgot to add this to RNG schema as well.
According to documentation, the value match the dateTime type.

docs/schemas/domain.rng

@@ -1222,6 +1222,11 @@
               <text/>
             </attribute>
           </optional>
+          <optional>
+            <attribute name="passwdValidTo">
+              <data type="dateTime"/>
+            </attribute>
+          </optional>
         </group>
         <group>
           <attribute name="type">
@@ -1260,6 +1265,11 @@
               <text/>
             </attribute>
           </optional>
+          <optional>
+            <attribute name="passwdValidTo">
+              <data type="dateTime"/>
+            </attribute>
+          </optional>
           <interleave>
             <zeroOrMore>
               <element name="channel">