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>

Documentation/devicetree/bindings/ata/brcm,sata-brcm.txt

@@ -9,6 +9,7 @@ Required properties:
 			"brcm,bcm7445-ahci"
 			"brcm,bcm-nsp-ahci"
 			"brcm,sata3-ahci"
+			"brcm,bcm63138-ahci"
 - reg                : register mappings for AHCI and SATA_TOP_CTRL
 - reg-names          : "ahci" and "top-ctrl"
 - interrupts         : interrupt mapping for SATA IRQ