ARM: dts: cx92755: align UART node name with bindings

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

Acked-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/20230123151527.369285-1-krzysztof.kozlowski@linaro.org


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