soc/intel: Intel graphics driver scans generic bus

This change allows for Intel graphics devices to use drivers/generic/gfx
driver to populate ACPI SSDT table for common graphics related devices
and methods.

BUG=b:142237145
TEST=On sarien_cml add generic/gfx to the devicetree and device is
     enumerated and correct SSDT ASL is observed.

Change-Id: Ibc86a88687ac860ebef19a4b68af64fd50d12b8e
Signed-off-by: Mathew King <mathewk@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36042
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/soc/intel/common/block/graphics/graphics.c

@@ -118,6 +118,7 @@ static const struct device_operations graphics_ops = {
 	.init			= graphics_soc_init,
 	.ops_pci		= &pci_dev_ops_pci,
 	.write_acpi_tables	= graphics_soc_write_acpi_opregion,
+	.scan_bus		= scan_generic_bus,
 };
 
 static const unsigned short pci_device_ids[] = {