arm64: dts: qcom: sm6375-pdx225: Add volume down GPIO key

Add the required nodes to enable the volume down key on the Sony
Xperia 10 IV.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230303-topic-sm6375_features0_dts-v2-14-708b8191f7eb@linaro.org