[POWERPC] use winbond libata instead of ide driver for pseries CD drives

Change the default for the built-in IDE on p610/p615/p630 from
ide to libata. libata has better error handling and the drive can
recover when hald does its CD media polling.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>