dt-bindings: phy: Document BCM63138 compatible string

Document the compatible string "brcm,bcm63138-sata-phy" as a valid
compatible string describing the standard Broadcom SATA PHY block.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/brcm-sata-phy.txt

@@ -8,6 +8,7 @@ Required properties:
      "brcm,iproc-nsp-sata-phy"
      "brcm,phy-sata3"
      "brcm,iproc-sr-sata-phy"
+     "brcm,bcm63138-sata-phy"
 - address-cells: should be 1
 - size-cells: should be 0
 - reg: register ranges for the PHY PCB interface