mb/google/brya: Mark the WWAN device as an UntrustedDevice

The ChromiumOS kernel has the ability to restrict devices to their own
IOMMU security domains when ACPI passes this property to a device
downstream of a PCIe RP.

BUG=b:215424986
TEST=verified the property is found and WWAN is restricted to its own
IOMMU domain as expected.

Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Change-Id: I1717c0976d1d961772245fd420368fe5a9c1262e
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61628
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>

src/mainboard/google/brya/Kconfig

@@ -14,6 +14,7 @@ config BOARD_GOOGLE_BRYA_COMMON
 	select DRIVERS_INTEL_PMC
 	select DRIVERS_INTEL_SOUNDWIRE
 	select DRIVERS_INTEL_USB4_RETIMER
+	select DRIVERS_PCIE_GENERIC
 	select DRIVERS_SOUNDWIRE_ALC5682
 	select DRIVERS_SOUNDWIRE_MAX98373
 	select DRIVERS_SPI_ACPI

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

@@ -166,6 +166,10 @@ chip soc/intel/alderlake
 				.clk_req = 5,
 				.flags = PCIE_RP_LTR | PCIE_RP_AER,
 			}"
+			chip drivers/pcie/generic
+				register "is_untrusted" = "1"
+				device pci 0 on end
+			end
 		end	#PCIE6 WWAN
 		device ref pcie_rp8 on
 			# Enable SD Card PCIE 8 using clk 3