[PATCH] powerpc: dont allow old RTC to be selected

Now powerpc uses the generic RTC stuff we should not enable the old RTC.
Doing so will result in hangs at boot.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>