dt-bindings: phy: Add binding doc for Stingray PCIe PHY

Add binding document for Stingray PCIe PHYs for both PAXB and PAXC based
root complex

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>