dt-bindings: usb: Add missing properties used in examples

With 'unevaluatedProperties' support implemented, the following warnings
are generated in the usb examples:

Documentation/devicetree/bindings/usb/intel,keembay-dwc3.example.dt.yaml: usb: usb@34000000: Unevaluated properties are not allowed ('reg' was unexpected)
Documentation/devicetree/bindings/usb/snps,dwc3.example.dt.yaml: usb@4a030000: Unevaluated properties are not allowed ('reg' was unexpected)

Add the missing property definitions.

Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Ahmad Zainie <[e-mail]>
Cc: Felipe Balbi <[e-mail]>
Cc: [e-mail]
Signed-off-by: Rob Herring <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/usb/intel,keembay-dwc3.yaml

···
   compatible:
     const: intel,keembay-dwc3
 
+  reg:
+    maxItems: 1
+
   clocks:
     maxItems: 4
 

Documentation/devicetree/bindings/usb/snps,dwc3.yaml

···
         - const: synopsys,dwc3
           deprecated: true
 
+  reg:
+    maxItems: 1
+
   interrupts:
     description:
       It's either a single common DWC3 interrupt (dwc_usb3) or individual
···
       anyOf:
         - enum: [bus_early, ref, suspend]
         - true
+
+  iommus:
+    maxItems: 1
 
   usb-phy:
     minItems: 1