arm64: dts: bitmain: lowercase unit addresses

Unit addresses are expected to be lower case.  Pointed also by W=1
builds:

  Warning (simple_bus_reg): /soc/serial@5801A000: simple-bus unit address format error, expected "5801a000"

Link: https://lore.kernel.org/r/20230712074611.35952-1-krzysztof.kozlowski@linaro.org


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