dt-bindings: ata: add compatible string for iProc AHCI controller

The Broadcom iProc SoCs have AHCI compliant SATA controller. This
patch adds common compatible string for AHCI SATA controller on
iProc SoCs.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>