dt-bindings: usb: add documentation for aspeed udc

Add device tree binding documentation for the Aspeed USB2.0 Device
Controller.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neal Liu <neal_liu@aspeedtech.com>
Link: https://lore.kernel.org/r/20220523030134.2977116-4-neal_liu@aspeedtech.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/aspeed,ast2600-udc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (c) 2020 Facebook Inc.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/usb/aspeed,ast2600-udc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ASPEED USB 2.0 Device Controller
+
+maintainers:
+  - Neal Liu <neal_liu@aspeedtech.com>
+
+description: |+
+  The ASPEED USB 2.0 Device Controller implements 1 control endpoint and
+  4 generic endpoints for AST260x.
+
+  Supports independent DMA channel for each generic endpoint.
+  Supports 32/256 stages descriptor mode for all generic endpoints.
+
+properties:
+  compatible:
+    enum:
+      - aspeed,ast2600-udc
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/aspeed-clock.h>
+    udc: usb@1e6a2000 {
+        compatible = "aspeed,ast2600-udc";
+        reg = <0x1e6a2000 0x300>;
+        interrupts = <9>;
+        clocks = <&syscon ASPEED_CLK_GATE_USBPORT2CLK>;
+        pinctrl-names = "default";
+        pinctrl-0 = <&pinctrl_usb2bd_default>;
+    };