ata: Fix SC1200 dependencies

The SC1200 is a SoC based on the Geode GX1 32-bit x86 processor, so
its drivers are only needed on this architecture, except for build
testing purpose.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Tejun Heo <tj@kernel.org>