doc: dt-binding: cdns-salvo-phy: add binding doc

Add Cadence SALVO PHY binding doc, this PHY is a legacy module,
and is only used for USB3 and USB2.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>