ARM: dts: nuvoton: align SPI NOR node name with dtschema

The node names should be generic and SPI NOR dtschema expects "flash".

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220407143127.295008-1-krzysztof.kozlowski@linaro.org