spi: dt-bindings: nuvoton,wpcm450-fiu: Fix warning in example (missing reg property)

Add missing "reg = <0>;" property to the flash@0 node in the example.

Fixes: dd71cd4d ("spi: Add Nuvoton WPCM450 Flash Interface Unit (FIU) bindings")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221129102225.3598044-3-j.neuschaefer@gmx.net


Signed-off-by: Mark Brown <broonie@kernel.org>