[POWERPC] disable PReP and EFIKA during make oldconfig

New boards should not be enabled per default.
Disable EFIKA and PReP per default.
Anyone who really needes the new code can enable it during make oldconfig.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>