dt-bindings: phy: Document BCM7216 SATA PHY compatible string

Define "brcm,bcm7216-sata-phy" as a new compatible string for the
Broadcom SATA3 PHY.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>