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>