[POWERPC] Turn on ATA_SFF so we get SATA_SVW back in defconfigs

This enables CONFIG_ATA_SFF in the defconfigs that are intended to
work on a G5 powermac, i.e. g5_defconfig and ppc64_defconfig.  Since
the support for the SATA cell in the K2 chipset is provided by the
sata_svw.c driver, and that depends on CONFIG_ATA_SFF, we need to turn
that and CONFIG_SATA_SVW back on so we can get to the hard disk on G5s.

Signed-off-by: Paul Mackerras <paulus@samba.org>