ata: dwc_ahci: Fix support for other platforms

The dwc_ahci driver use platform specific defines, place the platform
specific code behind a ifdef CONFIG_ARCH_OMAP2PLUS to allow build and
use of the driver on Rockchip platform.

Fixes: 02a4b429 ("drivers: block: dwc_ahci: Implement a driver for Synopsys DWC sata device")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>