arm64: dts: amazon: drop deprecated serial device_type

The device_type property is deprecated by Devicetree specification and
bindings do not allow it.

Link: https://lore.kernel.org/r/20230123151547.369791-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>