arm64: dts: synaptics: align UART node name with bindings

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

  berlin4ct-dmp.dtb: uart@d000: $nodename:0: 'uart@d000' does not match '^serial(@.*)?$'

Link: https://lore.kernel.org/r/20230123151540.369690-1-krzysztof.kozlowski@linaro.org


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