[POWERPC] Add rtas_service_present() helper

To test for the existence of an RTAS function, we typically do:

   foo_token = rtas_token("foo");
   if (foo_token == RTAS_UNKNOWN_SERVICE)
      return;

Add a rtas_service_present method, which provides a more conventional
boolean interface for testing the existence of an RTAS method.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/rtas.c

@@ -303,6 +303,12 @@ int rtas_token(const char *service)
 }
 EXPORT_SYMBOL(rtas_token);
 
+int rtas_service_present(const char *service)
+{
+	return rtas_token(service) != RTAS_UNKNOWN_SERVICE;
+}
+EXPORT_SYMBOL(rtas_service_present);
+
 #ifdef CONFIG_RTAS_ERROR_LOGGING
 /*
  * Return the firmware-specified size of the error log buffer