arm64: dts: qcom: sm8550: Use the correct BWMON fallback compatible

Use the correct fallback compatible for the BWMONv4 with merged global and
monitor register spaces.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230304-topic-ddr_bwmon-v3-7-77a050c2fbda@linaro.org