dt-bindings: phy: Add Stingray USB PHY binding document

Add DT binding document for Stingray USB PHY.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>