ahci: st: Only build for ST-Micro h/w

This device is designed specifically to run on ST-Microelectronics'
hardware. To ensure no attempts are made to run on anything incompatible
we add a dependency on ST architecture

Suggested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>