dt-bindings: ata: rename brcm, sata-brcmstb.txt to brcm, sata-brcm.txt

Renames the dt bindings document of Broadcom STB ahci driver to
common Broadcom ahci driver bindings document

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>