ata: fix i.MX AHCI driver dependencies

The ahci_imx driver is only needed on Freescale i.MX platforms so
don't let it be built on other platforms, except for build test
purpose.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Richard Zhu <r65037@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>