arm64: dts: broadcom: drop deprecated serial device_type

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

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230123151616.369963-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>