arm: mainstone_defconfig: disable IDE subsystem

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

Cc: Daniel Mack <daniel@zonque.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>