dt-bindings: usb: dwc3-imx8mp: Use the correct name for child node "snps, dwc3"

File snps,dwc3.yaml describes the schema of Synopsys DesignWare USB3
Controller, it directly or indirectly contains "$ref: usb.yaml". So the
node name of "snps,dwc3" must start with "usb". Otherwise, the following
warning will be displayed:

Documentation/devicetree/bindings/usb/fsl,imx8mp-dwc3.example.dt.yaml: \
dwc3@38100000: $nodename:0: 'dwc3@38100000' does not match '^usb(@.*)?'
        From schema: Documentation/devicetree/bindings/usb/snps,dwc3.yaml

In addition, replace "type: object" with "$ref: snps,dwc3.yaml#". Ensure
that all properties of the child node comply with snps,dwc3.yaml.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210329072714.2135-2-thunder.leizhen@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/fsl,imx8mp-dwc3.yaml

@@ -52,11 +52,8 @@ properties:
 # Required child node:
 
 patternProperties:
-  "^dwc3@[0-9a-f]+$":
-    type: object
-    description:
-      A child node must exist to represent the core DWC3 IP block
-      The content of the node is defined in dwc3.txt.
+  "^usb@[0-9a-f]+$":
+    $ref: snps,dwc3.yaml#
 
 required:
   - compatible
@@ -87,7 +84,7 @@ examples:
       dma-ranges = <0x40000000 0x40000000 0xc0000000>;
       ranges;
 
-      dwc3@38100000 {
+      usb@38100000 {
         compatible = "snps,dwc3";
         reg = <0x38100000 0x10000>;
         clocks = <&clk IMX8MP_CLK_HSIO_AXI>,