mb/google/myst: Enable iommu

Enable iommu in devicetree for myst in order to allow kernel to load and
initialize IOMMU.

Bug=b:276805280
TEST=builds

Signed-off-by: Jon Murphy <jpmurphy@google.com>
Change-Id: I94e93afe775b070253464a9d187ad6c028d1b811
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74177
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

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

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 chip soc/amd/phoenix
 	device domain 0 on
+		device ref iommu on end
 		device ref lpc_bridge on
 			chip ec/google/chromeec
 				device pnp 0c09.0 alias chrome_ec on end