arm64: dts: qcom: msm8996: drop unused slimbus dmas

Bindings document only two DMA channels.  Linux driver also does not use
remaining rx2/tx2.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220928152027.489543-7-krzysztof.kozlowski@linaro.org