powerpc/85xx: disable IDE subsystem in stx_gp3_defconfig

This patch disables deprecated IDE subsystem in stx_gp3_defconfig
(no IDE host drivers are selected in this config so there is no valid
reason to enable IDE subsystem itself).

Cc: Scott Wood <oss@buserror.net>
Cc: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Scott Wood <oss@buserror.net>