ARM: dts: broadcom: align UART node name with bindings

Bindings expect UART/serial node names to be "serial".

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230123151616.369963-2-krzysztof.kozlowski@linaro.org


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