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>

Documentation/devicetree/bindings/phy/cdns,salvo-phy.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright (c) 2020 NXP
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/phy/cdns,salvo-phy.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Cadence SALVO PHY
+
+maintainers:
+  - Peter Chen <peter.chen@nxp.com>
+
+properties:
+  compatible:
+    enum:
+      - nxp,salvo-phy
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: salvo_phy_clk
+
+  power-domains:
+    maxItems: 1
+
+  "#phy-cells":
+    const: 0
+
+required:
+  - compatible
+  - reg
+  - "#phy-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/firmware/imx/rsrc.h>
+
+    usb3phy: usb3-phy@5b160000 {
+        compatible = "nxp,salvo-phy";
+        reg = <0x5b160000 0x40000>;
+        clocks = <&usb3_lpcg 4>;
+        clock-names = "salvo_phy_clk";
+        power-domains = <&pd IMX_SC_R_USB_2_PHY>;
+        #phy-cells = <0>;
+    };