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

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

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20230123151533.369533-1-krzysztof.kozlowski@linaro.org


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