ata: ahci_platform: Add DT compatible for Synopsis DWC AHCI controller

Add compatible string "snps,dwc-ahci", which should be used
for Synopsis Designware SATA cores. e.g. on TI OMAP5 and DRA7 platforms.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>