iommu: qcom-smmu: add qcom generic compatible

With the exception of SDM845, most other Qualcomm SoCs have the Qualcomm
specific (but not SoC) specific SMMU compatible string. Add it here so
we can match those without having to add individual SoCs to the list
here.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>