mb/google/myst: Enable internal graphics

Enable internal graphics on the phoenix soc for myst projects.

BUG=b:275900162
TEST=builds

Signed-off-by: Jon Murphy <jpmurphy@google.com>
Change-Id: Ia6ef1ca07b9af491c7d937be5cef4f051852e486
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74104
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

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

@@ -1,6 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 chip soc/amd/phoenix
 	device domain 0 on
+		device ref gpp_bridge_a on  # Internal GPP Bridge 0 to Bus A
+			device ref gfx on end # Internal GPU (GFX)
+		end
 		device ref iommu on end
 		device ref lpc_bridge on
 			chip ec/google/chromeec