Finally I tested this latest patchset 34 on my A88XM-E with A10-6700 installed, and for me it's the opposite regarding IOMMU: if I apply this patchset as is, I get " AMD-Vi: Can not reserve memory region 0-80000 for mmio. AMD-Vi: This is a BIOS bug. Please contact your hardware vendor " at the beginning of Linux Kernel 4.15 booting (Linux Mint 19.2 LiveUSB) and, a few seconds later, iommu_disable kernel panic.

However, with IOMMU enabled, same Linux kernel is booting perfectly! And I see this IOMMU device at lspci. My .config - https://pastebin.com/5xrX7eqf , cat ./src/mainboard/asus/a88xm-e/config_seabios
###
### SeaBIOS custom configuration for ASUS A88XM-E
###
# CONFIG_MEGASAS is not set
# CONFIG_NVME is not set
#

Although csb_patcher.sh from CB:33509 has been applied, its' changes do not affect IOMMU function, so I assume that IOMMU should be working for you as well with a small change below:

diff --git a/src/mainboard/asus/a88xm-e/devicetree.cb b/src/mainboard/asus/a88xm-e/devicetree.cb
index 501d0f9995..aa00a5654a 100644
--- a/src/mainboard/asus/a88xm-e/devicetree.cb
+++ b/src/mainboard/asus/a88xm-e/devicetree.cb
@@ -26,7 +26,7 @@ chip northbridge/amd/agesa/family15tn/root_complex
 
                        chip northbridge/amd/agesa/family15tn # PCI side of HT root complex
                                device pci 0.0 on  end # Root Complex
-                               device pci 0.2 off end # IOMMU
+                               device pci 0.2 on  end # IOMMU
                                device pci 1.0 on  end # Internal Graphics
                                device pci 1.1 on  end # Internal Audio (iGPU)
                                device pci 2.0 on  end # PCIEX16

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