You can disable SMMU for PCIe controller-5 by removing the following two lines from the device-tree file
iommus = <&smmu TEGRA_SID_PCIE5>;
dma-coherent;
Having said that, I genuinely feel that your code should be checked once as to why it can’t work with SMMU enabled for PCIe. It looks to me that you might not be using dma_alloc_* / dma_map_* APIs which isn’t a good thing to do from the system security point of view as the PCIe controller (based on accesses from endpoint) can access any random memory location in the system with SMMU disabled.