dt-bindings: Remove more cases of 'allOf' containing a '$ref'

Another round of 'allOf' removals that came in this cycle.

json-schema versions draft7 and earlier have a weird behavior in that
any keywords combined with a '$ref' are ignored (silently). The correct
form was to put a '$ref' under an 'allOf'. This behavior is now changed
in the 2019-09 json-schema spec and '$ref' can be mixed with other
keywords. The json-schema library doesn't yet support this, but the
tooling now does a fixup for this and either way works.

This has been a constant source of review comments, so let's change this
treewide so everyone copies the simpler syntax.

Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -52,21 +52,18 @@ properties:
 
   vhub-vendor-id:
     description: vhub Vendor ID
-    allOf:
-      - $ref: /schemas/types.yaml#/definitions/uint32
-      - maximum: 65535
+    $ref: /schemas/types.yaml#/definitions/uint32
+    maximum: 65535
 
   vhub-product-id:
     description: vhub Product ID
-    allOf:
-      - $ref: /schemas/types.yaml#/definitions/uint32
-      - maximum: 65535
+    $ref: /schemas/types.yaml#/definitions/uint32
+    maximum: 65535
 
   vhub-device-revision:
     description: vhub Device Revision in binary-coded decimal
-    allOf:
-      - $ref: /schemas/types.yaml#/definitions/uint32
-      - maximum: 65535
+    $ref: /schemas/types.yaml#/definitions/uint32
+    maximum: 65535
 
   vhub-strings:
     type: object
@@ -90,18 +87,15 @@ properties:
 
           manufacturer:
             description: vhub manufacturer
-            allOf:
-              - $ref: /schemas/types.yaml#/definitions/string
+            $ref: /schemas/types.yaml#/definitions/string
 
           product:
             description: vhub product name
-            allOf:
-              - $ref: /schemas/types.yaml#/definitions/string
+            $ref: /schemas/types.yaml#/definitions/string
 
           serial-number:
             description: vhub device serial number
-            allOf:
-              - $ref: /schemas/types.yaml#/definitions/string
+            $ref: /schemas/types.yaml#/definitions/string
 
 required:
   - compatible