dt-bindings: usb: snps,dwc3: Fix "snps,hsphy_interface" type

The "snps,hsphy_interface" is string, not u8. Fix the type.

Fixes: 389d7765 ("dt-bindings: usb: Convert DWC USB3 bindings to DT schema")
Cc: stable <stable@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230515172456.179049-1-marex@denx.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -287,7 +287,7 @@ properties:
     description:
       High-Speed PHY interface selection between UTMI+ and ULPI when the
       DWC_USB3_HSPHY_INTERFACE has value 3.
-    $ref: /schemas/types.yaml#/definitions/uint8
+    $ref: /schemas/types.yaml#/definitions/string
     enum: [utmi, ulpi]
 
   snps,quirk-frame-length-adjustment: