ARM: dts: dm814x: align UART node name with bindings

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

Link: https://lore.kernel.org/r/20230123151536.369623-1-krzysztof.kozlowski@linaro.org


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