arm64: dts: qcom: sdm845-shift-axolotl: enable SLPI

Enable the SLPI DSP on the SHIFTPHONES SHIFT6mq phone with a
Qualcomm SDM845 SoC.

Signed-off-by: Dylan Van Assche <me@dylanvanassche.be>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230406173148.28309-6-me@dylanvanassche.be