Use the absent IOMMU module for the reference boards.

Change-Id: Idca3b7fc44734774aee047f3113ff8a172b1e187
diff --git a/BUILD.gn b/BUILD.gn
index 16be10e..f4c684a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -56,6 +56,7 @@
   origin_address = "0x80000000"
   boot_flow = "//src/boot_flow:linux"
   console = "//src/arch/aarch64/pl011"
+  iommu = "//src/iommu:absent"
   gic_version = 3
   gicd_base_address = "0x2f000000"
   gicr_base_address = "0x2f100000"
@@ -72,6 +73,7 @@
   origin_address = "0x40001000"
   boot_flow = "//src/boot_flow:linux"
   console = "//src/arch/aarch64/pl011"
+  iommu = "//src/iommu:absent"
   gic_version = 3
   gicd_base_address = "0x08000000"
   gicr_base_address = "0x080A0000"
@@ -88,6 +90,7 @@
   origin_address = "0x80000"
   boot_flow = "//src/boot_flow:linux"
   console = "//project/reference/rpi:mini_uart"
+  iommu = "//src/iommu:absent"
   gic_version = 2
   heap_pages = 60
   max_cpus = 4