dt-bindings: Add Broadcom STB USB PHY binding document

Add support for bcm7216 and bcm7211

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