mb/google/skyrim/variants/baseboard: enable iommu

With IOMMU disabled, kernel complains that 'IOMMUv2 functionality not
available on this system'. Enable iommu in devicetree for skyrim proto
board in order to allow kernel to load and initialize IOMMUv2.

BUG=b:232750390
TEST=Boot to Chrome OS on skyrim board, and
     grep dmesg for "AMD IOMMUv2 loaded and initialized"

Change-Id: I2f10f5eda8083335619a34c44df253b8e5a8572c
Signed-off-by: Jason Glenesk <Jason.Glenesk@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65190
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>

src/mainboard/google/skyrim/variants/baseboard/devicetree.cb

@@ -69,6 +69,7 @@ chip soc/amd/sabrina
 				device pci 00.0 on end
 			end
 		end
+		device ref iommu on end
 		device ref gpp_bridge_1 on end # SD
 		device ref gpp_bridge_2 on end # NVMe