dt-bindings: phy: realtek: Add Realtek DHC RTD SoC USB 3.0 PHY

Document the USB PHY bindings for Realtek SoCs.
Realtek DHC (digital home center) RTD SoCs support DWC3 XHCI USB
controller and using USB 3.0 PHY transceiver.
Signed-off-by: Stanley Chang <stanley_chang@realtek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230725033318.8361-5-stanley_chang@realtek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/phy/realtek,usb3phy.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2023 Realtek Semiconductor Corporation
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/phy/realtek,usb3phy.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Realtek DHC SoCs USB 3.0 PHY
+
+maintainers:
+  - Stanley Chang <stanley_chang@realtek.com>
+
+description: |
+  Realtek USB 3.0 PHY support the digital home center (DHC) RTD series SoCs.
+  The USB 3.0 PHY driver is designed to support the XHCI controller. The SoCs
+  support multiple XHCI controllers. One PHY device node maps to one XHCI
+  controller.
+
+  RTD1295/RTD1619 SoCs USB
+  The USB architecture includes three XHCI controllers.
+  Each XHCI maps to one USB 2.0 PHY and map one USB 3.0 PHY on some
+  controllers.
+  XHCI controller#0 -- usb2phy -- phy#0
+                    |- usb3phy -- phy#0
+  XHCI controller#1 -- usb2phy -- phy#0
+  XHCI controller#2 -- usb2phy -- phy#0
+                    |- usb3phy -- phy#0
+
+  RTD1319/RTD1619b SoCs USB
+  The USB architecture includes three XHCI controllers.
+  Each XHCI maps to one USB 2.0 PHY and map one USB 3.0 PHY on controllers#2.
+  XHCI controller#0 -- usb2phy -- phy#0
+  XHCI controller#1 -- usb2phy -- phy#0
+  XHCI controller#2 -- usb2phy -- phy#0
+                    |- usb3phy -- phy#0
+
+  RTD1319d SoCs USB
+  The USB architecture includes three XHCI controllers.
+  Each xhci maps to one USB 2.0 PHY and map one USB 3.0 PHY on controllers#0.
+  XHCI controller#0 -- usb2phy -- phy#0
+                    |- usb3phy -- phy#0
+  XHCI controller#1 -- usb2phy -- phy#0
+  XHCI controller#2 -- usb2phy -- phy#0
+
+properties:
+  compatible:
+    enum:
+      - realtek,rtd1295-usb3phy
+      - realtek,rtd1319-usb3phy
+      - realtek,rtd1319d-usb3phy
+      - realtek,rtd1619-usb3phy
+      - realtek,rtd1619b-usb3phy
+
+  reg:
+    maxItems: 1
+
+  "#phy-cells":
+    const: 0
+
+  nvmem-cells:
+    maxItems: 1
+    description: A phandle to the tx lfps swing trim data provided by
+      a nvmem device, if unspecified, default values shall be used.
+
+  nvmem-cell-names:
+    items:
+      - const: usb_u3_tx_lfps_swing_trim
+
+  realtek,amplitude-control-coarse-tuning:
+    description:
+      This adjusts the signal amplitude for normal operation and beacon LFPS.
+      This value is a parameter for coarse tuning.
+      For different boards, if the default value is inappropriate, this
+      property can be assigned to adjust.
+    $ref: /schemas/types.yaml#/definitions/uint32
+    default: 255
+    minimum: 0
+    maximum: 255
+
+  realtek,amplitude-control-fine-tuning:
+    description:
+      This adjusts the signal amplitude for normal operation and beacon LFPS.
+      This value is used for fine-tuning parameters.
+    $ref: /schemas/types.yaml#/definitions/uint32
+    default: 65535
+    minimum: 0
+    maximum: 65535
+
+required:
+  - compatible
+  - reg
+  - "#phy-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    usb-phy@13e10 {
+        compatible = "realtek,rtd1319d-usb3phy";
+        reg = <0x13e10 0x4>;
+        #phy-cells = <0>;
+
+        nvmem-cells = <&otp_usb_u3_tx_lfps_swing_trim>;
+        nvmem-cell-names = "usb_u3_tx_lfps_swing_trim";
+
+        realtek,amplitude-control-coarse-tuning = <0x77>;
+    };