[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>