dt-bindings: ata: Document BCM63138 compatible string

Document the compatible string "brcm,bcm63138-ahci" as a valid
compatible string for the standard Broadcom AHCI controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>