dt-bindings: usb: add documentation for aspeed usb-vhub

Add device tree binding documentation for the Aspeed USB 2.0 Virtual HUb
Controller.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

Documentation/devicetree/bindings/usb/aspeed,usb-vhub.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (c) 2020 Facebook Inc.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/usb/aspeed,usb-vhub.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ASPEED USB 2.0 Virtual Hub Controller
+
+maintainers:
+  - Benjamin Herrenschmidt <benh@kernel.crashing.org>
+
+description: |+
+  The ASPEED USB 2.0 Virtual Hub Controller implements 1 set of USB Hub
+  register and several sets of Device and Endpoint registers to support
+  the Virtual Hub's downstream USB devices.
+
+  Supported number of devices and endpoints vary depending on hardware
+  revisions. AST2400 and AST2500 Virtual Hub supports 5 downstream devices
+  and 15 generic endpoints, while AST2600 Virtual Hub supports 7 downstream
+  devices and 21 generic endpoints.
+
+properties:
+  compatible:
+    enum:
+      - aspeed,ast2400-usb-vhub
+      - aspeed,ast2500-usb-vhub
+      - aspeed,ast2600-usb-vhub
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  aspeed,vhub-downstream-ports:
+    description: Number of downstream ports supported by the Virtual Hub
+    allOf:
+      - $ref: /schemas/types.yaml#/definitions/uint32
+      - default: 5
+        minimum: 1
+        maximum: 7
+
+  aspeed,vhub-generic-endpoints:
+    description: Number of generic endpoints supported by the Virtual Hub
+    allOf:
+      - $ref: /schemas/types.yaml#/definitions/uint32
+      - default: 15
+        minimum: 1
+        maximum: 21
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - interrupts
+  - aspeed,vhub-downstream-ports
+  - aspeed,vhub-generic-endpoints
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/aspeed-clock.h>
+    vhub: usb-vhub@1e6a0000 {
+            compatible = "aspeed,ast2500-usb-vhub";
+            reg = <0x1e6a0000 0x300>;
+            interrupts = <5>;
+            clocks = <&syscon ASPEED_CLK_GATE_USBPORT1CLK>;
+            aspeed,vhub-downstream-ports = <5>;
+            aspeed,vhub-generic-endpoints = <15>;
+            pinctrl-names = "default";
+            pinctrl-0 = <&pinctrl_usb2ad_default>;
+    };