dt-bindings: pinctrl: qcom,sm8150: Drop duplicate function value "atest_usb2"

The enum value "atest_usb2" appears twice. Remove the duplicate. The
meta-schema normally catches these, but schemas under "$defs" was not
getting checked. A fix for that is pending.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230418150613.1528233-1-robh@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>