ARM: dts: at91: 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>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220407143223.295344-1-krzysztof.kozlowski@linaro.org