arm64: dts: qcom: msm8996: Rename smmu nodes

Node names shouldn't include a vendor prefix and should whenever
possible use a generic identifier. Resolve this by renaming the smmu
nodes "iommu".

Reviewed-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>