dt-bindings: Update documentation for stingray SATA phy

Update BRCM SATA phy dt-binding documentation for stingray.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <rob@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>