sync clock with RDS time

Tested with my SansaClip+. I don't think this will need extra battery
but let me know if I am wrong.

Change-Id: I287dae134113e0f8a138af68f5087b8ea45b0f4c

manual/appendix/config_file_options.tex

@@ -370,6 +370,10 @@
       force fm mono
                 & off, on               & N/A\\
     }%
+    \opt{rds}{
+      sync RDS time
+                & off, on               & N/A\\
+    }%
 
     \bottomrule
   \end{longtable}

manual/configure_rockbox/time_and_date.tex

@@ -21,4 +21,9 @@ if voice support is enabled.
     }%\opt{recording,radio}
   }%\opt{alarm}
   \item[Time Format:] Choose 12 or 24 hour clock.
+  \opt{rds}{
+    \item[Sync RDS time:]
+        If this option is enabled, your clock will be kept in sync with the time provided
+        by Radio Data System (RDS) if the currently selected station provides it.
+  }%\opt{rds}
 \end{description}